Remote Sensing of Environment

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1 Remote Sensing of Environment 132 (2013) Contents lists vilble t SciVerse ScienceDirect Remote Sensing of Environment journl homepge: Effect of imge sptil nd spectrl chrcteristics on mpping semi-rid rngelnd vegettion using multiple endmember spectrl mixture nlysis (MESMA) K.R. Thorp,, A.N. French, A. Rngo b USDA-ARS, U.S. Arid Lnd Agriculturl Reserch Center, N Crdon Ln, Mricop, AZ 85138, United Sttes b USDA-ARS, Jornd Experimentl Rnge, P.O. Box 30003, MSC 3JER, NMSU, Ls Cruces, NM 88003, United Sttes rticle info bstrct Article history: Received 19 September 2012 Received in revised form 20 November 2012 Accepted 9 Jnury 2013 Avilble online xxxx Keywords: AVIRIS CAI Creosote Grss Hyperspectrl HyspIRI Imging spectroscopy Mesquite NDVI Nonphotosynthetic Sptil scle Spectr Encrochment of invsive shrubs into grsslnd res on rngelnds in the southwestern United Sttes thretens the vibility of livestock production nd cn severely lter hydrology nd biodiversity. Novel remote sensing technologies my provide useful informtion for monitoring nd remediting this thret. The objectives were to investigte multiple endmember spectrl mixture nlysis (MESMA) s n pproch to mp rngelnd vegettion using hyperspectrl remote sensing imgery nd to test the sensitivity of MESMA to lterntive imge sptil resolutions nd spectrl wvebnd combintions. Dt from two Airborne Visible/Infrred Imging Spectrometer (AVIRIS) overflights t the Jornd Experimentl Rnge in southwestern New Mexico were used in the nlysis. Endmember spectr were selected from librry of ground-bsed spectrl observtions collected with field spectrordiometer. A 4-endmember MESMA ws conducted for both AVIRIS imges t their ntive sptil resolutions using nm wvebnds from 422 to 2339 nm. Additionl MESMAs were conducted t 10 multiples of the imges' ntive sptil resolution nd for 6 lterntive combintions of spectrl wvebnd subregions. Mps of endmember frctionl cover for green shrub vegettion, nonphotosynthetic grss vegettion, nd bre soil were comprble to n erlier vegettion clssifiction mp of Jornd. MESMA frctionl cover estimtes for the green vegettion endmember were positively correlted with the normlized difference vegettion index (NDVI) with correltion coefficients (r) greter thn Correltion coefficients between the sum of the green nd nonphotosynthetic vegettion endmembers nd the cellulose bsorption index (CAI) were greter thn Correltion coefficients between MESMA frctionl green vegettion cover nd NDVI from independent multispectrl imges were greter thn Despite losses in sptil detil t corser imge sptil resolutions, MESMA results for imges with sptil resolution degrded by fctor of 10 (~150 m) were similr to ggregted results for MESMA t the ntive sptil resolution (~15 m). Additionlly, MESMA results were shown to be substntilly more sensitive to the spectrl wvebnds used in the nlysis s compred to the sptil resolution of the imges. Considered together, the MESMA results t Jornd indicted tht fine spectrl resolution with hyperspectrl remote sensing ws substntilly more importnt thn incrementl chnges in imge sptil scle from 15 m to 150 m. Published by Elsevier Inc. 1. Introduction Over the pst century, rngelnds in the northern Chihuhun Desert hve been severely degrded due to livestock overgrzing nd other humn ctivities (Hoyt, 2002). Drought nd climte chnge hve lso plyed role (Chehbouni et l., 2000). Severl focused studies of vegettion chnge t the Jornd Experimentl Rnge (Jornd) in southwestern New Mexico hve documented the encrochment of invsive shrubs, such s mesquite (Prosopis glndulos), creosote bush (Lrre tridentt), nd trbush (Flourensi cernu), into res once dominted by grss. These intrusions hve led to severe losses in grsslnd vegettion t Jornd (Buffington nd Herbel, 1965; Gibbens et l., 2005). Unsuccessful efforts to remedite grsslnds Corresponding uthor. Tel.: ; fx: E-mil ddress: kelly.thorp@rs.usd.gov (K.R. Thorp). by shrub removl suggest tht shrub invsion leds to soil erosion nd hydrologic ltertions, which impede the reestblishment of grss species (Rngo et l., 2005). Continued efforts to monitor nd remedite these threts to rngelnd environments will be bolstered by the vilbility of remote sensing technologies nd the development of novel remote sensing instrumenttion nd dt processing lgorithms. Previous pproches using eril photogrphy t Jornd hve been nrrowly focused nd lcking in spectrl informtion (Rngo et l., 2005). Comprehensive, rpid, nd repeted viewing of lrge rngelnd res will require deployment of sensors t high ltitude or from orbit. With the dvent of irborne hyperspectrl remote sensing instrumenttion such s AVIRIS (Green et l., 1998), nlyses using hyperspectrl dt to chrcterize rngelnd degrdtion nd monitor vegettion chnge re incresing (Asner nd Heidebrecht, 2002; Asner et l., 2000; Li et l., 2005; Mnsour et l., 2012; Yng et l., 2009). Further investigtion of /$ see front mtter. Published by Elsevier Inc.

2 K.R. Thorp et l. / Remote Sensing of Environment 132 (2013) these novel remote sensing techniques is required before the technology cn deliver on its promise to guide policy development nd support long-rnge plnning for better mngement of rngelnds. The primry dvntge of hyperspectrl remote sensing over current multispectrl techniques is its bility to resolve the reflectnce responses of imge fetures with fine spectrl detil, usully in hundreds of nrrow, contiguous spectrl wvebnds. Hyperspectrl observtions of homogeneous vegettion cnopies my demonstrte unique responses over certin wvebnd regions s compred to tht for other plnt species or soil bckgrounds (Thenkbil et l., 2000). In cses where vegettion cnopies re not homogeneous or where sptil resolution is too corse to void mixed pixels, spectrl mixture nlysis cn be used to resolve fine scle lnd surfce fetures. Spectrl mixture nlysis (SMA) is hyperspectrl dt processing pproch tht quntifies the proportions of lnd surfce fetures within mixed pixels using knowledge of ech feture's pure spectrl response or endmember. A mjor limittion of SMA is tht every imge pixel is unmixed using the sme endmember spectr. This is problemtic when feture is present only in certin portions of the imge or when feture exhibits substntil spectrl vrition tht is not well represented by one endmember spectrl observtion (Dennison nd Roberts, 2003). Multiple endmember spectrl mixture nlysis (MESMA) ddresses these problems by testing multiple combintions of endmembers nd endmember spectr for ech pixel in the imge (Roberts et l., 1998). Thus, MESMA increses the flexibility of simple SMA. As compred to common multispectrl pproches, the min dvntge of SMA nd MESMA is their bility to fcilitte remote sensing dt nlyses using full spectrum informtion. Imge sptil nd spectrl resolutions re expected to influence MESMA outcomes. However, the sensitivity of MESMA results to these fctors hs not been quntified. A better understnding of MESMA performnce constrints is informtive for the development of novel hyperspectrl imging systems. For exmple, the Ntionl Aeronutics nd Spce Administrtion (NASA) is currently developing the Hyperspectrl Infrred Imger (HyspIRI) nd is plnning for the instrument's deployment by stellite (hyspiri.jpl.ns.gov). HyspIRI will include n imging spectrometer observing from 380 nm to 2500 nm in 10 nm contiguous bnds nd multispectrl imger observing from 3 to 12 μm. These instruments will provide imges with sptil resolution of 60 m t ndir view ngle. Given these design chrcteristics, imges from other sensors cn be used to simulte nd evlute HyspIRI dt sets prior to lunch. Imges from the AVIRIS sensor re useful for evlutions of the HyspIRI imging spectrometer, becuse AVIRIS spectrl resolution is similr to HyspIRI (viris.jpl.ns.gov). Also, the sptil resolution of the AVIRIS sensor, flown t the typicl ltitude of 20 km, is 4 times finer thn tht proposed for HyspIRI, which llows for ssessments of the effects of HyspIRI's corser sptil resolution on dt nlysis. The first objective ws to investigte multiple endmember spectrl mixture nlysis (MESMA) s n pproch to mp rngelnd vegettion using hyperspectrl remote sensing dt, such s tht currently obtined with AVIRIS nd proposed for HyspIRI. The Jornd Experimentl Rnge ws the field site used for the nlysis. The second objective ws to test the sensitivity of MESMA results to degrdtions in imge sptil resolution nd to the use of lterntive spectrl wvebnds. The ltter objective supports design efforts for the HyspIRI sensor prior to lunch. 2. Mterils nd Methods 2.1. Jornd Experimentl Rnge The Jornd Experimentl Rnge (Jornd; jornd.nmsu.edu) ws estblished by the United Sttes Deprtment of Agriculture in 1912 to develop effective strtegies for mngement of livestock grzing lnds. It is 783 km 2 semirid rngelnd locted on the Jornd del Muerto Plin in the northern prt of the Chihuhun Desert in southwestern New Mexico, roughly 37 km northest of the city of Ls Cruces nd 40 km west of the White Snds Ntionl Monument (Fig. 1). Similr to the nlysis of Jornd vegettion provided by Gibbens et l. (2005), the nlysis reported herein focuses on the 582 km 2 Jornd plin west of the Sn Andres Mountins. Jornd hs been Ntionl Science Foundtion Long-Term Ecologicl Reserch site since Hvstd et l. (2000) provide detiled description of Jornd nd its historicl use s site for evlution of stellite remote sensing systems, including the JORNEX nd PROVE (Privette et l., 2000) experiments Airborne Imging Spectroscopy The AVIRIS imging spectrometer (Green et l., 1998) ws used to collect remote sensing imges over Jornd on severl dtes from 1997 to The present nlysis focused on two of those dtes: June 15, 2001 nd October 9, The instrument ws deployed using n irborne pltform flying pproximtely 20 km bove se level. AVIRIS collected rdiometric observtions between 380 nm nd 2500 nm using 224 unique detectors ech with nominl spectrl bndwidth of 10 nm. A whiskbroom scnning mirror with 12 Hz scnning rte ws used to obtin cross-trck sptil smples. Five flight lines were required to obtin full coverge of Jornd on ech cquisition dte. Preliminry dt processing steps, including geometric nd rdiometric correction, were completed by the AVIRIS Dt Fcility. Imges were georeferenced using geometric look-up tble constructed using dt from the nvigtion equipment bord the ircrft. Imges were lso clibrted to spectrl rdince. Further dt preprocessing, including tmospheric correction nd dditionl georeferencing, ws conducted in-house. Atmospheric correction ws ccomplished with the Second Simultion of Stellite Signl in the Solr Spectrum (6S) lgorithm (Kotchenov et l., 2006). Bsed on the cross-trck sptil smpling intervl of 0.87 millirdins, the viewer zenith ngle ws clculted t intervls of 25 pixels cross the imge swth, nd 6S prmeters for the remining pixels were interpolted. Since ll imges were collected long due north or due south flight line, viewer zimuth ngles were set t 90 nd 270 for pixels est nd west of the flight line, respectively. Solr zenith nd zimuth ngles were computed t the time nd position of the ircrft midwy through the dt cquisition period for ech imge (Red nd Andres, 2004). Atmospheric profiles for pressure, temperture, nd wter density from ground-level to the sensor-level were obtined from the Ntionl Center for Environmentl Prediction (NCEP) Renlysis dt set, provided by the NOAA/OAR/ESRL PSD, Boulder, Colordo, USA ( The dt set contins long-term, high-resolution climte informtion for the North Americn domin (Mesinger et l., 2006). The NCEP dt nerest in time nd spce to the AVIRIS overflights t Jornd were used to specify the 6S prmeters for tmospheric profile. The 6S defult Desert Model with visibility of 16 km ws used to define the erosol profile. Spectrl conditions were ssumed to be monochromtic; 6S ws provided the wvelength t the center of ech of AVIRIS wvebnd. Lmbertin ground reflectnce ws ssumed. Given the complexity of correcting multiple bnds nd scenes, Python script ( ws developed to conduct 6S simultions nd tmosphericlly correct the AVIRIS imges. Automted methods t the AVIRIS Dt Fcility provided mjor help with imge geometric corrections. However, dditionl corrections were needed to properly mosick the five imges on ech dt collection dte. Imges were therefore corrected further by selecting ground control points from n orthophoto provided by the United Sttes Geologicl Survey (USGS). Approximtely 50 points per imge were required for improving the imge registrtion. Imges were then georegistered using polynominl wrping

3 122 K.R. Thorp et l. / Remote Sensing of Environment 132 (2013) Fig. 1. Site mp for the Jornd Experimentl Rnge in southwestern New Mexico, USA. Orthophoto dt were obtined from the U.S. Geologicl Survey (erthexplorer.usgs.gov). with nerest neighbor resmpling. To prepre for mosicking the imges were resmpled to the verge sptil resolution of the originl imges: 16.6 m for the 2001 imges nd 15.5 m for the 2002 imges. This best preserved the sptil resolution of the originl imges. The five imges from ech dt collection dte were then mosicked,ndthemosickedimgeswereclippedtotheboundries of Jornd. These tsks were completed using the Environment for Visulizing Imges (ENVI) softwre (Version 4.8, Exelis Visul Informtion Solutions, Boulder, CO) Endmember Spectr Endmember spectr for conducting MESMA on the AVIRIS imges were selected from ground-bsed observtions of rngelnd vegettion using portble field spectrordiometer (FieldSpec Pro, Anlyticl Spectrl Devices, Inc., Boulder, CO). Rdiometric mesurements were collected in 2151 nrrow wvebnds from 350 to 2500 nm with bndwidth of 1 nm. The instrument ws equipped with 25 field-of-view fiber optic cble, which ws positioned pproximtely 1.5 m bove the ground. Spectrl mesurements typiclly occurred round the time of solr noon. Frequent rdiometric observtions of clibrted, 99% Spectrlon pnel (Lbsphere, Inc., North Sutton, New Hmpshire) were used to chrcterize solr irrdince throughout the dt collection period. Cnopy reflectnce fctors in ech wvebnd were computed s the rtio of the cnopy rdince over the corresponding timeinterpolted vlue for solr irrdince. Ground-bsed rdiometric mesurements were collected on My 5 nd My 8 in 2001, pproximtely six weeks prior to the AVIRIS overflight on June 15. In 2002, ground-bsed observtions were collected on October 4, five dys prior to the AVIRIS overflight on October 9. Rdiometric mesurements were collected long predefined trnsects t loctions dominted by mesquite, grss, creosote bush nd trbush. A fifth trnsect ws locted in n re tht ws trnsitioning from grss to mesquite. Between 125 nd 200 scns were collected t regulr intervls (every few meters) long ech trnsect. The fetures within the sensor's field-of-view were documented for ech scn. The uthors credit Dr. Jerry Ritchie ( ), former Reserch Soil Scientist t USDA-ARS nd long-term contributor t Jornd, for his efforts to collect nd orgnize these spectrl dt sets. To select endmembers for MESMA, spectrl scns tht were documented s mixture of bre soil nd vegettion were eliminted, becuse MESMA requires pure spectr for ech feture clss. Remining scns were seprted into five clsses, including mesquite, creosote bush, trbush, grss, nd bre soil. These scns were ssessed grphiclly by plotting the dt nd numericlly by clculting the well-known normlized difference vegettion index (NDVI) nd plotting histogrms of this index. These inspections were useful for finding nd eliminting erroneous entries. Further culling of endmember spectr ws ccomplished with MESMA techniques. A Python progrm hndled the djustment of the spectrometer dt to mtch the AVIRIS wvebnds Multiple Endmember Spectrl Mixture Anlysis (MESMA) An open-source plug-in for the ENVI softwre clled Visuliztion nd Imge Processing for Environmentl Reserch (VIPER) Tools (Version 1.5, ws used to implement MESMA for these dt sets. The tool fcilittes the cretion nd mngement of endmember spectrl librries, the sttisticl nlysis of seprbility mong spectrl clsses, nd the implementtion of MESMA lgorithms. MESMA bsed on one, two, or three unique spectrl librries

4 K.R. Thorp et l. / Remote Sensing of Environment 132 (2013) Percent Reflectnce * b Percent Reflectnce * c d Percent Reflectnce * e * f Wvelength (nm) Wvelength (nm) Fig. 2. Endmember spectr used for multiple endmember spectrl mixture nlysis (MESMA) of AVIRIS imges t the Jornd Experimentl Rnge. Spectr of green shrub vegettion in ) My 2001 nd b) October 2002, nonphotosynthetic grss vegettion in c) My 2001 nd d) October 2002, nd bre soil in e) My 2001 nd f) October 2002 werecollectedwithfield spectrordiometer. cn be conducted. Ech spectrl librry represents one endmember, but cn contin multiple spectr. MESMA then unmixes ech pixel in the imge by testing ll possible combintions of spectr included in these spectrl librries. Finl endmember frctions re ssigned to ech pixel bsed on the endmember combintion tht gives the lowest root men squred error (RMSE) between modeled nd observed spectrl responses. Ech MESMA includes n dditionl endmember for shde, which cn be either provided by the user or defulted to zero (photometric shde). The ltter option ws used for ll nlyses in this study. The minimum llowble endmember frction, mximum llowble endmember frction, nd mximum llowble shde frction were set to vlues of 0.05, 1.05, nd 0.8, respectively. By llowing endmember frctions slightly greter thn 1.00 nd slightly less thn 0.00, MESMA ws permitted to consider models tht fit well in spite of endmember frctions tht were slightly physiclly unrelistic. Otherwise, these potentilly worthwhile models would hve been eliminted from considertion. No RMSE or residul constrints were used for MESMAs in this study. Five unique spectrl librries, one for ech of the five feture clsses, were creted with the scns from the ground-bsed spectrordiometer in ech yer. Due to possible sesonl vrition in vegettion phenology, the ground-bsed spectrl dt from the two yers were hndled seprtely. Five two-endmember MESMAs were then conducted on ech AVIRIS imge s n pproch to identify the scns from ech clss tht were best ble to unmix the imge. Following these nlyses, only the endmember spectr tht were ble to clssify greter thn 0.1% of the totl imge re were retined. This procedure reduced the number of possible endmember combintions nd thus reduced the computtionl time required for subsequent four-endmember MESMA. Endmember spectr remining fter the two-endmember nlysis were tested for spectrl seprbility mong clsses using the VIPER Tools squre rry computtion, which reports severl sttisticl

5 124 K.R. Thorp et l. / Remote Sensing of Environment 132 (2013) b km km Fig. 3. Multiple endmember spectrl mixture nlysis (MESMA) result for ) June 15, 2001 AVIRIS imgery nd b) October 9, 2002 AVIRIS imgery over the Jornd Experimentl Rnge. Results re displyed s n RGB imge where R, G, nd B represent frctionl cover of nonphotosynthetic grss, green shrub vegettion, nd bre soil, respectively. metrics on the use of ech spectrum to unmix ech of the other spectr in spectrl librry. From the squre rry dt, the verge RMSE for ech spectrum to model other spectr within ech of the five clsses ws clculted. If n endmember spectrum ws better t modeling spectr outside its own clss thn within it, the spectrum ws visully inspected nd considered for elimintion from the nlysis. After these efforts to fine tune the endmember spectr, the remining endmembers were incorported into three spectrl librries representing green vegettion (shrubs), nonphotosynthetic vegettion (grss), nd bre soil for ech yer (Fig. 2). The NDVI for mesquite, trbush, nd creosote bush spectr ws not less thn 0.42, so these spectr were merged to represent green vegettion endmember clss. The NDVI for grss spectr did not exceed 0.16, so grss spectr were incorported into n endmember clss for nonphotosynthetic vegettion. A third endmember clss ws estblished for bre soil. Using these three endmember clsses plus fourth for shde, fourendmember MESMA ws conducted for the AVIRIS imges in ech yer Wvebnd Elimintion Preliminry nlyses demonstrted the sensitivity of MESMA results to the spectrl wvebnds included, nd severl issues necessitted Tble 1 Evlution of 2001 nd 2002 MESMA endmember frctionl cover results t Jornd SW NW Bis b r c NDVI d CAI e NDVI f CAI g NDVI h NDVI i EM1 GV EM2 NPV EM3 Soil EM4 Shde EM1+EM Multiple endmember spectrl mixture nlysis (MESMA); southwest (SW); northwest (NW); normlized difference vegettion index (NDVI); cellulose bsorption index (CAI); endmember (EM); green vegettion (GV); nonphotosynthetic vegettion (NPV). b Averge frctionl cover bis between 2001 nd 2002 MESMA results. c Person correltion between 2001 nd 2002 MESMA results. d Person correltion between 2001 MESMA result nd 2001 AVIRIS NDVI. e Person correltion between 2001 MESMA result nd 2001 AVIRIS CAI. f Person correltion between 2002 MESMA result nd 2002 AVIRIS NDVI. g Person correltion between 2002 MESMA result nd 2002 AVIRIS CAI. h Person correltion between 2001 MESMA result nd 2001 SW IKONOS NDVI. i Person correltion between 2001 MESMA result nd 2001 NW IKONOS NDVI. tht some wvebnds be removed from the nlysis. For exmple, wvebnds flling outside the optiml rnges of the three detectors in the ground-bsed spectrordiometer were usully chrcterized by low signl-to-noise rtio. Also, due to limited input dt, the 6S tmospheric corrections of the AVIRIS dt were unresonble for some wvebnds, prticulrly those t tmospheric bsorption bnds. Severl grphicl nd quntittive pproches were used to identify the wvebnds tht needed to be removed from the nlysis. These procedures eliminted nerly hlf of the wvebnds from the dt sets, so tht MESMA ws conducted using only 113 of the originl 224 wvebnds. These 113 bnds were locted minly within the visible nd NIR region from 422 to 1088 nm (VIS/NIR) nd within the SWIR region from 1513 to 1712 nm (SWIR1) nd from 2030 to 2339 nm (SWIR2) (Fig. 2) MESMA Evlution Remote sensing is ttrctive for rngelnd monitoring, becuse it cn rpidly nd comprehensively mp lrge lnd res using smll frction of the time nd lbor needed for ground-bsed reconnissnce. The irony is the difficulty in evluting the results of remote sensing dt nlyses, becuse comprehensive ground-bsed dt sets for this purpose re typiclly prohibited by the cost nd lbor required to collected them. For the cse of Jornd, intensive ground reconnissnce in 1998 nd set of eril photogrphs collected in 1996 were used by (Gibbens et l., 2005) to construct mps tht delineted the loction of dominnt vegettion types cross Jornd. However, frctionl vegettion cover ws not quntified in these mps, nd the vegettion cover delinetions were not resolved to 15 m sptil resolution. Thus, while the historicl mps provided essentil context for lnd cover delinetion t Jornd, they were insufficient for vlidtion of the MESMA results derived from AVIRIS. As n lterntive, three pproches were dopted to ensure quntittive consistency nd resonbleness: 1) compring the MESMA results for 2001 with tht for 2002, 2) compring the MESMA results with known spectrl indices, nd 3) compring the 2001 MESMA results with n nlysis of two IKONOS imges t 4 m sptil resolution collected over subsection of Jornd on My 23, MESMA Annul Comprison Efforts to preserve the originl sptil resolution of the AVIRIS dt resulted in MESMA imges with slightly different sptil scles: 16.6 m for the 2001 imges nd 15.5 m for the 2002 imges.

6 K.R. Thorp et l. / Remote Sensing of Environment 132 (2013) Therefore, results could not be directly compred, pixel by pixel. Insted, set of 10,000 points ws rndomly generted within the Jornd boundry nd the MESMA results for 2001 nd 2002 were smpled t these loctions. MESMA endmembers were ssessed by clculting the verge bis, Person's correltion coefficient, nd Spermn's rnk correltion coefficient between the results for the two yers MESMA nd spectrl indices Person's correltion coefficients were clculted between the frctionl cover of ech MESMA endmember nd two spectrl indices, including the well-known normlized difference vegettion index (NDVI) nd the cellulose bsorption index (CAI) developed by Ngler et l. (2003). The NDVI (Rouse et l., 1973) ws clculted s NDVI ¼ R NIR R RED R NIR þ R RED where R NIR is the spectrl reflectnce in the ner-infrred region nd R RED is the spectrl reflectnce in the red region. Bsed on the third nd fourth Lndst TM bnds, the AVIRIS dt in 9 bnds from 635 to 683 nm nd in 13 bnds from 779 to 894 nm were verged to estimte the red nd NIR wvebnd reflectnces, respectively. Focused in the SWIR2 region, the CAI ws clculted s CAI ¼ 100:0 R 2030 þ R 2210 R 2: where R 2030, R 2210,ndR 2100 re nrrow-bnd reflectnces t 2030, 2210, nd 2100 nm, respectively. The AVIRIS reflectnces in the wvebnd nerest to these required wvelengths were used to clculte CAI. The CAI is commonly used to estimte crop residue or plnt litter cover. Positive CAI vlues indicte the presence of cellulose, nd vlues cn rech6.0forfullcropresiduecover(ngler et l., 2003) km Fig. 4. Clssifiction mp of creosote bush (blck), mesquite (grey), nd trbush (white) species bsed on multiple endmember spectrl mixture nlysis (MESMA) of n October 9, 2002 AVIRIS imge over the Jornd Experimentl Rnge. ð1þ ð2þ MESMA nd IKONOS Imges Two IKONOS imges (GeoEye, Herdon, VA) were collected over Jornd on My 23, 2001, which ws 23 dys prior to the AVIRIS overflight in tht yer. Ech imge covered pproximtely 118 km 2, one focusing on the northwestern portion of Jornd nd the other focusing on the southwestern portion. The multispectrl IKONOS sensor observed blue rdition from 445 to 540 nm, green rdition from 507 to 595 nm, red rdition from 632 to 697 nm, nd NIR rdition from 757 to 850 nm. Ech of these wvebnds ws observed with 4 m sptil resolution, pproximtely 4 times finer thn the AVIRIS dt. IKONOS imges were georeferenced by the imge provider. IKONOS imges were clibrted to percent reflectnce using n empiricl line method bsed on reflectnce from the 2001 AVIRIS dt. The IKONOS dt were first degrded to 16 m, nd two of the AVIRIS flightlines were coregistered to the degrded IKONOS dt. AVIRIS spectr within the four IKONOS wvebnds were verged to generte four-bnd AVIRIS imge with the sme sptil nd spectrl scle s the IKONOS dt. Both dt sets were smpled on pixelby-pixel bsis to develop n empiricl reltionship between AVIRIS reflectnces nd IKONOS digitl numbers. These empiricl reltionships were used to clibrte the IKONOS dt t 4 m sptil resolution, nd n IKONOS NDVI (Eq. 1) ws computed. A four-endmember MESMA ws conducted for the AVIRIS spectr within the IKONOS imge boundries. Person's correltion coefficients were clculted between ech MESMA endmember nd the IKONOS NDVIs ggregted from 4 m to 16 m. This procedure permitted quntittive evlution of MESMA results using n independent dt set collected t sptil resolution 4timesfiner thn the AVIRIS dt MESMA for Degrded Imges Degrdtions in imge sptil or spectrl resolution were expected to influence MESMA results. For exmple, MESMA results my differ when using 15 m AVIRIS dt s compred to the proposed 60 m HyspIRI dt. Results could lso differ if sensor did not observe the sme wvebnd regions s the AVIRIS sensor. To evlute these issues, MESMA ws performed using the endmember spectr (Fig. 2) to unmix sptilly nd spectrlly degrded AVIRIS imges Sptil Degrdtions The effects of imge sptil degrdtion on MESMA results were evluted t 10 multiples of the ntive sptil resolutions of the 2001 nd 2002 AVIRIS imges over Jornd. Thus for 2001, evlutions were conducted t the following sptil resolutions: 16.6 m, 33.2 m, 49.8 m, 66.4 m, 83.0 m, 99.6 m, m, m, m, nd m. Similrly for 2002, evlutions were conducted t the following sptil resolutions: 15.5 m, 31.0 m, 46.5 m, 62.0 m, 77.5 m, 93.0 m, m, m, m, nd m. To generte truth imges for comprison purposes, the MESMA results for ech AVIRIS imge t its ntive sptil resolution were lso ggregted to these 10 sptil resolutions. These truth imges were nlyzed using the pproch of Woodcock nd Strhler (1987) to understnd the locl vrince for imge fetures t Jornd. A 3 3 filter convolution ws used to clculte the locl stndrd devition of the MESMA results for ech endmember t ech sptil resolution. The convolution result ws then sptilly verged within the Jornd boundry to summrize the locl stndrd devition t ech sptil resolution. This procedure ws used to summrize the loss of imge sptil content t Jornd s imge sptil resolution becme incresingly corse. To ssess MESMA performnce t corser sptil resolutions, the originl 2001 nd 2002 AVIRIS imges were lso degrded to ech of the 10 sptil resolutions. A 4-endmember MESMA, using the endmember spectr in Fig. 2, ws then conducted for ech degrded imge. Results for the MESMAs on the degrded AVIRIS imges were compred to the degrded truth imges from the MESMA t

7 126 K.R. Thorp et l. / Remote Sensing of Environment 132 (2013) b Locl Stndrd Devition Sptil Resolution (m) Sptil Resolution (m) EM1-GV EM2-NPV EM3-Soil Shde Fig. 5. Locl stndrd devition versus sptil resolution of MESMA endmember frctionl cover estimtes t Jornd on ) June 15, 2001 nd b) October 9, Results were generted using the method of Woodcock nd Strhler (1987) to sptilly degrde the MESMA endmembers (EM) for green vegettion (GV), nonphotosynthetic vegettion (NPV), bre soil, nd shde. the ntive sptil resolution. Results were ssessed by clculting the verge bis between these imges for ech endmember t ech sptil resolution. Also, the number of pixels with differences rnging from 0% to 1%, from 1% to 10%, nd from 10% to 120% ws clculted to ssess the level of disgreement between imges Spectrl Degrdtions The effects of imge spectrl chrcteristics on MESMA results were evluted by lterntively removing the VIS/NIR, SWIR1, nd SWIR2 wvebnd regions from the MESMA nlysis. These nlyses were conducted on both the 2001 nd 2002 AVIRIS imges t their ntive sptil resolutions. Six nlyses were conducted, including 1) MESMA without the SWIR2 region, 2) MESMA without the SWIR1 region, 3) MESMA without the VIS/NIR region, 4) MESMA using only the SWIR2 region, 5) MESMA using only the SWIR1 region, nd 6) MESMA using only the VIS/NIR region. Results for these MESMAs were compred to tht from the MESMA tht included ll three spectrl regions. Similr to the results for sptil resolution degrdtions, the effects of imge spectrl chrcteristics on MESMA were ssessed by clculting the verge bis between imges nd clculting the number of pixels with differences rnging from 0% to 1%, from 1% to 10%, nd from 10% to 120%. 3. Results nd Discussion 3.1. MESMA Results Efforts to develop endmember spectr from the set of ground-bsed spectrl reflectnce dt resulted in 30 spectr for the 2001 dt set, including 6 green vegettion spectr (ll mesquite), 10 nonphotosynthetic grss spectr, nd 14 bre soil spectr (Fig. 2). For the 2002 dt set, totl of 57 spectr were selected, including 22 green vegettion spectr, 15 nonphotosynthetic grss spectr, nd 20 bre soil spectr. Of the green vegettion spectr selected in 2002, there were 8 creosote bush spectr, 7 mesquite spectr, nd 7 trbush spectr. However, s demonstrted by grphicl pproches (Fig. 2b), spectrl seprbility mong the these three shrub clsses could not be chieved due to similrity mong the scns. Okin et l. (2001) corroborted this result, showing tht MESMA ws unble to relibly retrieve vegettion type in rid nd semirid environments where vegettion cover ws less thn 30%. In the present study, spectrl seprbility ws more evident mong the spectr for green vegettion, nonphotosynthetic grss, nd bre soil s compred to spectr for unique shrub species (Fig. 2). Results of the 4-endmember MESMA for the 2001 (Fig. 3) nd 2002 (Fig. 3b) AVIRIS imges were summrized using three-bnd Tble 2 Averge frctionl cover bis between 1) endmember frctionl cover from MESMAs on the sptilly degrded AVIRIS imges nd 2) endmember frctionl cover from the truth imges, generted by sptilly degrding the MESMA result t the ntive imge sptil resolution. Results re shown for June 15, 2001 (left) nd October 9, 2002 (right) imge dt. June 15, 2001 Imges October 9, 2002 Imges SR EM1 EM2 EM3 EM4 SR EM1 EM2 EM3 EM4 (m) GV NPV Soil Shde (m) GV NPV Soil Shde Multiple endmember spectrl mixture nlysis (MESMA); Sptil Resolution (SR); Endmember (EM); Green vegettion (GV); Nonphotosynthetic vegettion (NPV).

8 K.R. Thorp et l. / Remote Sensing of Environment 132 (2013) Percent of Pixels c Percent of Pixels 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% b d Sptil Resolution (m) Sptil Resolution (m) 0%-1%Different 1%-10%Different 10%-120%Different Fig. 6. Percent of imge pixels tht were from 0% to 1%, 1% to 10%, nd 10% to 120% different when compring 1) endmember frctionl cover from multiple endmember spectrl mixture nlyses (MESMA) on the sptilly degrded 2001 AVIRIS imges nd 2) endmember frctionl cover from the truth imges, generted by sptilly degrding the 2001 MESMA result t the ntive 16.6 m imge sptil resolution. Results re shown for ) green vegettion, b) nonphotosynthetic vegettion, c) bre soil, nd d) shde endmembers. RGB imge where R, G, nd B represented the frctionl cover of nonphotosynthetic grss, green shrub vegettion, nd bre soil, respectively. Subjective comprisons of the the MESMA results with the Jornd vegettion clssifiction mps provided by Gibbens et l. (2005) demonstrted tht the MESMA results were resonble. The west centrl portion of Jornd ws dominted by mesquite dunes in 1998 (Gibbens et l., 2005), condition rising from grdul replcement of tobos (Pleurphis mutic)nd blck grm(boutelou eriopod)grss species in the preceding century. MESMA results for AVIRIS imges in 2001 nd 2002 lso demonstrted dominnce of green shrub vegettion in this re. An re tht ws trnsitioning from grss species to mesquite ly long the southwestern border of Jornd. Both MESMA results showed dominnce of nonphotosynthetic grss in this re with encroching green shrub vegettion. Ptches of grss dominted res from the northwestern corner of Jornd through the center were evident in both the MESMA results nd the clssifiction mp of Gibbens et l. (2005). The northern nd northestern portions of Jornd were dominted by mesquite snd hills in 1998 (Gibbens et l., 2005). MESMA results for AVIRIS imges collected in 2001 nd 2002 lso showed dominnce of green shrub vegettion in this re. Visul comprisons of the ptterns in MESMA results demonstrted some similrities between 2001 nd 2002 (Fig. 3). Smpling these two MESMA imges for quntittive comprisons showed tht the endmembers for green shrub vegettion, nonphotosynthetic grss vegettion, bre soil, nd shde were similr, with Person's correltion coefficients of 0.41, 0.66, 0.67, 0.39 nd verge frctionl cover bises of 0.08, 0.10, 0.11, nd 0.13, respectively (Tble 1). Spermn's correltion coefficients (not shown) were not substntilly different from Person's correltion coefficients. Positive correltions were found for ll endmembers between the two yers, nd reltively high correltions were found for nonphotosynthetic vegettion nd bre soil endmembers. Positive bises for nonphotosynthetic vegettion nd bre soil endmembers with the negtive bis for green vegettion showed tht lower green vegettion cover ws quntified in the 2001 Tble 3 Averge frctionl cover bis between 1) endmember frctionl cover from MESMAs on the AVIRIS imges with key spectrl wvebnds excluded nd 2) endmember frctionl cover from the truth imges, which were the MESMA results using full spectrum dt. Results re shown for June 15, 2001 (left) nd October 9, 2002 (right) imge dt. June 15, 2001 imges October 9, 2002 imges EM1 EM2 EM3 EM4 EM1 EM2 EM3 EM4 GV NPV Soil Shde GV NPV Soil Shde No SWIR No SWIR No VIS/NIR Only SWIR Only SWIR Only VIS/NIR Multiple endmember spectrl mixture nlysis (MESMA); Endmember (EM); Green vegettion (GV); Nonphotosynthetic vegettion (NPV); Shortwve Infrred (SWIR); Visible/Ner-infrred (VIS/NIR).

9 128 K.R. Thorp et l. / Remote Sensing of Environment 132 (2013) Percent of Pixels c Percent of Pixels 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% no SWIR2 no no only only only SWIR1 VIS/NIR SWIR2 SWIR1 VIS/NIR Wvebnd Modifiction b d no SWIR2 no no only only only SWIR1 VIS/NIR SWIR2 SWIR1 VIS/NIR Wvebnd Modifiction 0%-1%Different 1%-10%Different 10%-120%Different Fig. 7. Percent of imge pixels tht were from 0% to 1%, 1% to 10%, nd 10% to 120% different when compring 1) endmember frctionl cover from multiple endmember spectrl mixture nlyses (MESMA) on the 2001 AVIRIS imge with key spectrl wvebnds excluded nd 2) endmember frctionl cover from the truth imge, which ws the 2001 MESMA result using full spectrum dt. Results re shown for ) green vegettion, b) nonphotosynthetic vegettion, c) bre soil, nd d) shde endmembers. MESMA s compred to the 2002 MESMA. Wether records t Jornd reported 15 mm nd 125 mm of rinfll in the three-month period preceding the 2001 nd 2002 AVIRIS overflights, respectively. Thus, higher green vegettion cover in October 2002 ws likely the result of soil wter rechrge from the summer monsoon. Despite this importnt physicl explntion, differences my hve lso resulted from errors in the tmospheric correction of the two AVIRIS imges nd from differences in MESMA clcultions due to the use of independent ground-bsed spectrl dt sets. Also, since the shde endmember showed the gretest bsolute frctionl cover bis (0.13) nd the lowest correltion coefficient (0.39) between the two yers, the modeled differences in shde frctionl cover likely ffected the cover frctions for the other endmembers. These considertions highlight need for cution in the use of MESMA results. Due to the sensitivity of MESMA results to shde ssumptions nd spectrl chrcteristics of the imges nd endmember spectr, the technique my be most useful for reltive comprisons of frctionl cover rther thn bsolute frctionl cover estimtes. With correltion coefficients of 0.73 nd 0.58 in 2001 nd 2002, respectively, the NDVI from AVIRIS dt ws well correlted with the green vegettion endmember from MESMAs on the AVIRIS dt (Tble 1). These correltions were positive s expected, becuse the NDVI is sensitive to green biomss. Correltions between the CAI nd the green vegettion endmember were much weker compred to tht for NDVI. The nonphotosynthetic grss endmember ws positively correlted with the CAI with correltion coefficients of 0.56 nd 0.40 in 2001 nd 2002, respectively. This greement highlighted the sensitivity of the CAI to cellulose in the nonphotosynthetic grss species t Jornd nd demonstrted tht MESMA ws resonbly ble to mp this vegettion chrcteristic. The CAI lso exhibited strong negtive correltions with the bre soil endmember in both yers. As cellulose incresed with incresing vegettion cover, bre soil cover ws expected to decrese, resulting in the negtive correltions. Summing the cover frction for the green vegettion nd nonphotosynthetic vegettion endmembers nd correlting to CAI, the correltion coefficients were 0.59 nd 0.60 in 2001 nd 2002, respectively. These correltions were greter thn tht between CAI nd nonphotosynthetic vegettion lone. Thus, CAI ws sensitive to the combined effect of green nd nonphotosynthetic vegettion cover. The NDVI ws not sensitive to this combintion. These correltions between the MESMA endmembers nd spectrl indices help to justify the MESMA results by demonstrting expected reltionships. With correltion coefficients of 0.71 nd 0.57 for northwestern nd southwestern subsets of Jornd, respectively, NDVI from independent IKONOS imges ws well correlted with the green vegettion endmember from MESMAs on the 2001 AVIRIS dt (Tble 1). This mens the frctionl green vegettion cover provided by MESMA ws comprble to tht estimted using common vegettion index with multispectrl imgery. An dvntge of MESMA using AVIRIS spectr ws tht it provided dditionl frctionl cover informtion for other endmembers, which ws not esily estimted using multispectrl dt. For exmple, with correltion coefficients lower thn 0.41, frctionl cover estimtes for bre soil nd for the sum of green nd nonphotosynthetic vegettion were not very well correlted with IKONOS NDVI dt. Frctionl cover estimtes for nonphotosynthetic vegettion were negtively correlted with NDVI for both IKONOS imges, resonble result considering tht nonphotosynthetic grss vegettion ws likely inversely proportionl with encroching green shrub vegettion cross Jornd. From the 2002 MESMA where endmember spectr were vilble for three shrub species, 20%, 74%, nd 6% of imge pixels were clssified using creosote bush, mesquite, nd trbush endmember spectr, respectively. This result ws fvorble, becuse mesquite ws likely the most dominte shrub cross the rnge, while creosote bush nd trbush were less common (Gibbens et l., 2005). Sptil ptterns of the clssifiction result for these shrubs roughly followed expected

10 K.R. Thorp et l. / Remote Sensing of Environment 132 (2013) results with mesquite more common in western nd northestern res, creosote bush more common in the northern nd estern regions, nd trbush more common in the southest (Fig. 4). However, the sptil ptterns re lso somewht rndom due to the difficulty in chieving spectrl seprbility mong the endmember spectr for the shrub species (Fig. 2b). Also, becuse the vegettion clssifiction mp in Fig. 4 focused only on the MESMA result for the green vegettion endmember, the result ws probbly most relible for the pixels in which the green vegettion endmember ws most dominnt. Frctionl cover for the green vegettion endmember ws dominnt (greter thn frctionl cover for the nonphotosynthetic vegettion nd bre soil endmembers) for only 18% of the imge pixels, nd 14% of the imge pixels were dominted by mesquite. By contrst, frctionl cover for the bre soil endmember ws dominte for 48% of the imge pixels. Plentiful bre soil nd reltively low creosote bush nd trbush coverge likely contributed to the difficulty in distinguishing between shrub species t Jornd. Spectrl mesurements of shded lower leves within shrub cnopy my hve helped identify unique shrub species, since trnsmission nd reflectnce within the cnopy is expected to depend on plnt morphology (Fitzgerld et l., 2005) Sptil Scle Effects With few exceptions, the locl stndrd devition of imge fetures t Jornd tended to decrese s the MESMA result t the ntive imge sptil resolution ws sptilly degrded (Fig. 5). In prticulr, the gretest loss of sptil detil occurred from the ntive resolution of the AVIRIS imges (~15 m) to round 50 m. This result highlighted n importnt difference between the dt collected by AVIRIS nd the 60 m dt expected from the HyspIRI sensor. For Jornd, 60 m resolution dt does not substntilly increse lnd cover informtion content compred to 150 m dt. In fct, even the 15 m irborne AVIRIS imges did not dequtely resolve the sptil fetures t Jornd, since pek in locl stndrd devition should be found t the optimum sptil resolution (Woodcock nd Strhler, 1987). This result demonstrted tht the optimum sptil resolution for resolving sptil fetures t Jornd is less thn 15 m, result supported by Pelgrum et l. (2000). The verge bis between endmember frctionl cover from MESMAs on the degrded AVIRIS imges nd tht from the truth imges ws not more thn nd 0.02 for the 2001 nd 2002 dt, respectively (Tble 2). Bises were smll, indicting tht MESMA results for sptilly degrded AVIRIS imges were comprble to sptil ggregtions of the MESMA results for imges t finer sptil scles. This mens tht corser imge resolution did not grossly effect MESMA performnce. Rther, it only ffected the sptil detil with which MESMA ws ble to resolve endmember frctionl cover. In compring the 2001 MESMAs for sptilly degrded imges with the truth imges, over 90% of imge pixels hd endmember frctionl cover vlues tht were less thn 10% different (Fig. 6). At sptil resolution of 33.2 m, round 45% of pixels hd endmember frctionl cover vlues less thn 1% different. The percentge of pixels less thn 1% different decresed to round 30% s the imge sptil resolution incresed to m. These results were consistent mong ll four endmembers. Similr ptterns were observed in compring the 2002 MESMAs for sptilly degrded imges with the 2002 truth imges (not shown) Spectrl Wvebnd Effects The verge bis between endmember frctionl cover from MESMAs on the AVIRIS imges with key wvebnds excluded were vrible depending on the test cse (Tble 3). Averge bises for MESMAs bsed on spectrlly degrded imges were often greter thn tht for sptilly degrded imges by fctor of 10 or 100 (Tbles 2 nd 3). Therefore, MESMA results were much more sensitive to the spectrl wvebnds used in the nlysis thn to the sptil resolution of the imges. Greter verge bises were typiclly found for the cses excluding VIS/NIR wvebnds, including only SWIR2 wvebnds, nd including only SWIR1 wvebnds (Tble 3). This result highlighted the importnce of the VIS/NIR wvebnds in guiding the MESMA towrd resonble results for this semirid rngelnd ppliction. Substntilly lrger bises were typiclly found for MESMAs bsed solely on reflectnce in the SWIR2 region from 2330 to 2339 nm. Only for the nonphotosynthetic vegettion did MESMA results for this test cse compre well with other test cses, likely due to the sensitivity of the SWIR2 wvebnds to the cellulose in grss vegettion (Ngler et l., 2003). With verge bises of nd in 2001 nd 2002, respectively, the SWIR2 bnds lone did not provide n effective wy to quntify green vegettion. However, the SWIR2 nd SWIR1 regions were more effective t quntifying green vegettion when used in combintion with VIS/NIR, s compred to ny of the three wvebnd regions lone. Similr conclusions cn be drwn from the percentge of pixels tht were from 0% to 1%, 1% to 10%, nd 10% to 120% different when compring endmember frctionl cover for MESMAs with spectrl wvebnds excluded with the truth MESMA bsed on full spectrum dt for 2001 (Fig. 7). Results for 2002 (not shown) were similr. Agreement for the green vegettion endmember ws poor when the VIS/NIR region ws removed from the nlysis. The best greement for green vegettion ws found when using combintion of VIS/NIR nd SWIR wvebnds to conduct MESMA nlyses. Agin, the higher sensitivity of MESMA to djustments in spectrl chrcteristics (Fig. 7) s opposed to sptil resolution chrcteristics (Fig. 6) ws found. 4. Conclusions Multiple endmember spectrl mixture nlysis (MESMA) ws n effective imge processing pproch for using hyperspectrl remote sensing dt to mp green vegettion, nonphotosynthetic vegettion, nd bre soil on semirid rngelnd: the Jornd Experimentl Rnge in southwestern New Mexico. However, MESMA results were sensitive to the chrcteristics of the imge spectr nd endmember spectr used in the nlysis. MESMA results were substntilly less sensitive to the sptil resolution of the input imges. Thus, MESMA is vluble dt nlysis tool for irborne nd stellite-bsed imging spectroscopy pplictions tht lck optimum imge sptil resolution to resolve the sptil fetures present in the scene. Neither AVIRIS nor the proposed HyspIRI sensor offer dequte imge sptil resolution to resolve sptil fetures t Jornd. However, sptil resolution is secondry in importnce to spectrl resolution for MESMA. Distinguishing between different shrub species t Jornd ws not supported using MESMA with the vilble endmember spectr. While MESMA techniques did distinguish between green shrub vegettion nd nonphotosynthetic grss vegettion, the techniques could not distinguish the three primry shrub species: mesquite, creosote bush, nd trbush. Selection of the spectrl wvebnds tht offered the best informtion for MESMA ws subjective nd not strightforwrd. Extensive understnding of nd experience with the input dt sets is required to obtin relible MESMA results. Future work will focus on the development of techniques to objectively select wvebnds for inclusion in the MESMA nlysis. Acknowledgements This reserch ws prtilly conducted with funds from the NASA Hedqurters, Science Mission Directorte, Reserch Opportunities in Spce nd Erth Sciences (ROSES) progrm entitled HyspIRI Preprtory Activities Using Existing Imgery. The uthors credit Dr. Jerry Ritchie ( ), former Reserch Soil Scientist t USDA-ARS nd longterm contributor t Jornd, for his efforts to collect nd orgnize the ground-bsed spectrl dt used in this study.