USE OF THE SRTM DEM AS A GEO-REFERENCING TOOL BY ELEVATION MATCHING

USE OF THE SRTM DEM AS A GEO-REFERENCING TOOL BY ELEVATION MATCHING J. A. Gonçalves a, A. M. Morgado b a Faculdade de Cêncas - Unversdade do Porto, Rua Campo Alegre, 4430-354 Porto, Portugal - jagoncal@fc.up.pt b Insttuto de Investgação Centífca Tropcal, Rua João de Barros, 27, 1300-319 Lsboa, Portugal - anamorgado@ct.pt Commsson II, WG II/7 KEY WORDS: DEM, Co-regstraton, Dgtal Photogrammetry, Matchng, Topographc Mappng ABSTRACT: Ths paper presents a methodology for planmetrc adjustment of elevaton data to the SRTM Dgtal Elevaton Model (SRTM- DEM), by correlaton. It can be appled to geo-reference any geographc dataset wth an elevaton component, such as rastersed maps wth contours or DEMs. The accuracy around 5 meters was acheved by ths geo-referencng method assessed usng elevaton data from Portugal derved by photogrammetrc technques. The method was then used to geo-reference old topographc maps of former Portuguese colones, whch were n local geodetc datums not well related to WGS-84. The consstency of planmetrc datum shfts determned was consstent wth the scales of the maps analysed. The method allowed for the use of these old maps wthout any extra feld work. 1. INTRODUCTION Ground control ponts (GCP) are frequently requred to georeference geographc datasets. In many stuatons feld surveys must be carred out, even when a hgh postonal accuracy s not requred. In remote areas the acquston of GCPs may be very expensve or very dffcult to carry out, wth a consequent ncreasng of costs. Archved satellte and aeral photography data sets can be an mportant source of ground control ponts for users that need them for geo-referencng purposes. Ths artcle descrbes a geo-referencng method wth applcatons on geographc nformaton, more specfcally on mages wth an altmetrc component. Ths applcaton s mportant for cartography on local datum wth an unknown relaton to the global system, such as the case of most hstorcal cartography of ex-portuguese colones, namely slands. The transformaton from the local datum usually nvolves nformaton acqured through feld work. Global datasets of georeferenced magery can consttute an adequate source to provde the necessary nformaton to the above mentoned applcatons. Presently a global coverage of Landsat ETM+ mages named Global Land Coverage Faclty (GLCF) s freely accessble to the publc. These are colour mages wth 15 meters spatal resoluton, resultng from data fuson between the panchromatc and three multspectral bands. They are rectfed to the UTM WGS84 coordnate system, wth a root mean square error (RMSE) of 40 meters (GLCF, 2004). Exstng archves of mages wth dfferent resolutons do not provde a hgher georeferencng precson when compared wth the one provded by sensor navgaton equpment. Ths s the case of hgh resoluton sensors such as Ikonos and Quckbrd, wth a spatal accuracy of 10 meters for Ikonos and slghtly worse for Quckbrd. In spte of ther commercal characterstc, many Quckbrd mages are currently avalable through Google Earth, wth easy pont dentfcaton and coordnate values. However, there s not a global coverage and the spatal accuracy s about 20meters decreasng wth hgh relef areas. In both cases (Google Earth and GLCF) the spatal uncertanty wll always be around dozens of meters. The alternatve presented n ths paper uses another dataset, also derved from remote sensng technques, wth an almost total global coverage: the Shuttle Radar Topographc Msson (SRTM) Dgtal Elevaton Model (DEM). Ths msson took place durng eleven days n February 2000 and provded an almost global altmetrc dataset between 56º and 60.25ºN lattudes. One arc second grds where automatcally produced through nterferometrc processng of the acqured SAR mages. Ths resoluton was only provded to the USA and made freely avalable wth a 3 arc seconds resoluton (93 meters at the Equator), as a generalsaton by pxel average (Rodrguez, et al, 2006). They consst of 1 by 1 degree fles, georeferenced to the WGS84, wth heghts above sea level as 16 bts ntegers. These dgtal elevaton models present some vod pxels at mountanous zones, manly due to the layover and shadow effects, common on SAR mages causng dffcultes on the phase unwrappng (Eneder, 2005). In extreme cases ths can be a lmtaton to the descrbed method; however ths does not occur n most stuatons. Fgure 1 shows two examples of extracted STRM-DEMs: Faal Island n Azores and Santo Antão Island n Cape Verde. In ths case there are a relatvely large number of vod pxels represented n black. Fgure 1. Examples of SRTM-DEMs 879

The expected precson for the SRTM DEM was 16 meters (90º) n heght and 10 meters n planmetry (Rabus, et al, 2003). These expectatons were exceeded especally on the vertcal component, wth many studes referrng 5 meters RMSE (8 meters at 90%) or better (Smth and Sandwell, 2003, Jacobsen, 2005, Gonçalves and Fernandes, 2005). Ths paper presents an analyss of the planmetrc precson of the SRTM DEM wth 3 arc second. Ths analyss s based on a correlaton technque between two altmetrc datasets: the SRTM DEM and a more detaled DEM resultng from medum scale cartography. Once the SRTM DEM precson s evaluated the same methodology s used to georeference the other altmetrc dataset usng an approxmate georeferencaton and the SRTM DEM. Ths method wll be descrbed n detal and t s applcable to georeference products wth an altmetrc component. It was appled to georeference nto WGS84, usng exstng contours from maps of several ex-portuguese Islands, where no nformaton of the transformaton parameters between the local datum and global datum was avalable. FOR =1 TO N lon_shft = lon()+dlon lat_shft = lat()+dlat Z2() = MDT2(lon_shft,lat_shft) NEXT CORR = CALC_CORREL (Z1,Z2) If(CORR > CORR_MAX) CORR_MAX = CORR NEXT lat NEXT lon The computaton of the maxmum correlaton value and the shft that orgnated that value would be determned. The search can be refned defnng a new narrower nterval and a smaller step. If the correlaton functon s regular and only a sngle maxmum s found n the area, the process wll converge to the requred value. Fgure 2 presents the solnes expressng the correlaton functon between the group of ponts and the SRTM DEM, of a specfc case study. The longtude (Δλ) and lattude (Δϕ) varatons are expressed n seconds. 2. PLANIMETRIC ADJUSTMENT OF A SET OF 3D POINTS TO THE SRTM-DEM 2.1 Method descrpton The approach followed n ths work s based on the correlaton between a heght dataset, normally coverng a regon of a few square klometres, and the SRTM-DEM. Both datasets are not spatally correctly regstered, due for example to a datum dfference. Snce both are n geographc coordnates t s expected that a shft would be enough to make them concde. The method s based on the search of that shft that provdes a maxmum correlaton between the two models. Other parameters, such as rotaton and scale, wll not be consdered but could also be computed n a smlar way. Consder A as the set of N ponts, {(λ,ϕ,h )} =1, n, (geographc coordnates and heght), to be shfted n order to be adjusted to the SRTM-DEM as the reference model. The correlaton coeffcent s gven by (Scheck, 1999): ρ = ' ' ( Z ) ( ) Z Z Z 2 ' ' ( Z Z ) ( Z Z ) Where Z s the Z (=1,...,N).values average, Z s he heght gven by the SRTM model at the (λ,ϕ ) poston, usng a blnear nterpolaton and Z ' s the average of those values. A value close to 1 represents a good correlaton between the two datasets. As a start the correlaton factor between the two datasets can be computed. A short longtude and lattude shft (Δλ, Δϕ) wll be appled to the coordnate ponts of A. Ths procedure s performed for a group of shfts, on a grd wth a sngle step. Extreme values of possble varaton n longtude and lattude as well as the step for those varatons are selected. Usng a language smlar to Basc, the algorthm can be descrbed as follows: FOR dlon=dlon_mn TO dlon_max STEP lon_step FOR dlat=dlat_mn TO dlat_max STEP lat_step 2 (1) Fgure 2. Graphc representaton of the correlaton coeffcent as a functon of the shfts appled to a data test (10Km 2 area) In ths example a dstnct maxmum value can be observed wth a value very close to 1 (more precsely 0.997) for a 3.4 longtude and 3.3 lattude (after teraton). The method does not work on flat terran, but n a general way the tests that wll be descrbed provded consderably good results, even n areas wth relatvely low relef. In the majorty of the cases, for areas between 10 Km 2 and 20 Km 2, a very well defned correlaton maxmum was acheved. For a good performance of the matchng process the ponts should be on areas wth some heght dfference, on slopes wth dfferent orentaton. In cases wth vegetaton, a correlaton lower than normal s expected, snce the SRTM-DEM presents the vegetaton heght n cases of dense vegetaton nstead of the terran representaton as the DEM derved from cartographc nformaton. However that does not prevent the method from workng. Once the correlaton s determned the hgher dfference between the heghts of the two DEMs can be computed. In case ths dfference s sgnfcantly dfferent from zero, that value wll ndcate a systematc dscrepancy due to the vegetaton. The method s appled to small areas, where slght dfferences of datum would result on an almost systematc shft. For large regons, local varatons wll be expected. In partcular cases when the cartographc projecton s unknown, the method can be appled wth the SRTM-DEM on a standard poston, for 880 2

example UTM and admt parameters to model the transformaton between the two DEMs (for example rotaton besdes translaton). 2.2 Precson evaluaton of the method usng data from Portuguese cartography Ths technque was evaluated applyng t to hghly accurate cartography of Portugal, wth a very well known georeference system. Sx dfferent areas (5 Km by 5 Km) were selected (Fgure 3) and the respectve contour lnes were extracted from 1:10 000 dgtal cartography, obtaned accordng to norms of the Geographc Portuguese Insttute, wth a 2 meters precson n planmetry and altmetry (IGP, 2007). Ths nformaton uses the natonal Gauss projecton and the local geodetc datum (Datum 73), wth precse known relaton to the WGS-84 (better than 0.5 meters). In all cases the dfferences between predcted and computed shfts dd not exceed 10 meters, wth RMSE of 5.2 and 3.8 meters n longtude and lattude. Ths fact shows the nonexstence of sgnfcant systematc errors on the SRTM and supports the applcablty of ths technque to georeference terran models assocated wth medum scales such as 1:25000. 2.3 Method evaluaton usng USA cartography avalable through the Internet In order to valdate the method usng data from dfferent locatons and sources, some altmetrc data samples were obtaned from web stes of cartographc producers. These samples consst of contour lnes fles (5Km by 5 Km) of sxty sx locatons from Texas (1:24 000 scale cartography USGS). Fgure 4 shows the locaton of the test areas. The method was appled to the ponts extracted from the contours and the respectve shfts were computed. These data are on NAD83 Datum, very smlar to the WGS84, therefore the expected shfts were almost zero. In four cases the correlaton obtaned was relatvely low, (below 0.9). These regons had heght ranges lower than 50 meters. A statstcal analyss was appled to the other sxty two areas and separately to the cases where the heght range s hgher than 100 meters. The results are presented n Table 2. Fgure 3. Test areas locaton consdered for the analyss of the method In each area the vertexes of the contour polylnes were extracted. Each group of ponts was transformed from map nto geographc coordnates (n the local geodetc datum) and the algorthm was developed n order to determne the shft to be performed to adjust each group to the SRTM-DEM. The dfference between Datum 73 and WGS84 s determned, usng a 7 parameters transformaton, allowng a precson hgher than 0.5 meters. (IGP, 2005). Table 1 presents the results obtaned, for the 6 locatons: the shft found through the adjustment (n arc seconds) and the error,.e, the dfference to the shft predcted by the datum transformaton. Ths error was converted to meters, takng nto account the 1 arc second dmenson at each area. The table also presents the heght range of the areas. Fgure 4. Locaton of the sxty sx areas to test the adjustment to the SRTM model The results showed a 5 meters precson, smlarly to the prevously presented n secton 2.2. Loc. Heght Shfts found Errors # range λ ( ) ϕ ( ) λ (m) ϕ (m) 1 429 3.48 3.10-6.2-0.3 2 273 3.58 3.19-9.6-2.9 3 426 3.28 3.11-0.8-1.1 4 203 3.60 3.28-1.2-6.4 5 223 3.44 3.27-5.3-5.9 6 322 3.26 3.02 0.8-0.5 RMSE 5.2 3.8 Errors n 62 cases (Δh > 50 m) Errors n 37 cases (Δh > 100 m) λ (m) ϕ (m) λ (m) ϕ (m) Mean 0.2 3.0 1.1 3.1 Std. dev. 5.1 6.8 3.9 4.8 RMSE 5.1 7.4 4.0 5.7 Table 2. Results found n the assessment of the method usng dgtsed elevaton data from the US. Table 1. Results found n the assessment of the method usng local dgtsed elevaton data. 881 3

3. APPLICATION IN THE GEOREFERENCIATION OF OLD MAPS An nterestng applcaton of ths method conssts of the possblty to georeference to a global coordnate system the dgtsed topographc maps. In case of old cartography there s usually no knowledge of the parameters relatng the geodetc map datum to the global datum. In actual projects of geographc nformaton analyss, such as change detecton studes, old cartography can be useful and n many cases the old cartography s stll the only one avalable. In ths paper Cape Verde, S. Tomé and Prncpe and East Tmor cartography produced durng the 60 s and 70 s, whle these countres were stll Portuguese colones, wll be used as study cases. In case of the Archpelagos dverse geodetc datums exst, almost one for each sland. Ths s justfed by the lack of enough vsblty between the slands to allow the connecton between the geodetc networks. In some cases some work was developed to fnd the parameters to convert between datums, however that nformaton s usually not ncluded n the lsts of coordnate systems of geographc nformaton processng software. Consderng small slands t was assumed that the transformaton between the local datum and the WGS84 system would consst of a constant shft between geographc coordnates. It s mportant to menton that makng use of a map at a certan scale the computed datum shftss wll not have a precson hgher than the related wth that scale. 3.1 São Tomé and Príncpe case study The S. Tomé and Príncpe cartography was produced durng the 60 s by Junta de Investgação do Ultramar, named nowadays Insttuto de Investgação Centífca Tropcal. Ths cartography was produced on the Mercator projecton system at the 1:25 000 scale, based on a local datum. Actually both slands present dfferent geodetc datums, besdes dfferent projectons wth dfferent parameters. Contour lnes were dgtsed on steep areas wth dfferent orentaton. The requred rgour s not hgh snce assurng that the vertexes of the dgtsed contours the ponts whch wll be used are postoned on the contour lnes. The workload s relatvely low, much less than the manual dgtsaton of the whole altmetry. Contour lnes of four dfferent locatons were dgtsed, coverng a 5 Km by 5 Km, one on each map. After the vertexes of the dgtsed contours were extracted and the converson nto geographc coordnates was performed the adjustment to the SRTM-DEM was then performed for the four locatons. The values found for the shftt whch maxmse the correlaton, as well as the correlaton value and the heght shft whch mnmses the quadratc average of the heght dfferences, are shown n Table 3. Locato Δλ ( ) Δϕ ( ) Corr Max ΔH n 1-20.2-2.0 0.9971 11 2-20.4-3.1 0.9800 18 3-20.5-2.9 0.9958 19 4-20.6-2.7 0.9900 21 Table 3. Datum shfts found for maps of S. Tomé sland The computed shfts have an average value of Δλ = 20.42, Δϕ= 2.65 and standard devaton, converted nto meters of σ λ = 5.9 m and σ ϕ = 14.7 m. The relatvely hgh value for the standard devaton n lattude can be explaned by varous reasons. The dense sland vegetaton, the hgh number of vod pxels on the SRTM DEM (partcularly on the S. Tomé Island) or the qualty of the cartography tself can explan such dfferences. However takng nto account the stuaton they can stll be adequate for the orgnal map scale (1:25 000). It should be mentoned the relatvely hgh values found for the shft n heght. The postve values show a systematc shft due to vegetaton, snce the heght values gven by the SRTM DEM correspond to the top of the dense vegetaton and the map heghts correspond to the terran. 3.2 Cape Verde case study The maps were acqured for a research project on bogeography by the Portuguese Bodversty Research Centre (CIBIO) n collaboraton wth the authors. The maps at 1:25 000 scale were dgtsed and georeferenced usng the map grd ponts. That grd s based on the UTM projecton, Hayford ellpsod, zone 27 n the case of the four frst slands, on zone 26 n the case of the other sx. As n the S. Tomé study case, contour lnes were dgtsed on selected lmted areas. Fgure 5 presents the Fogo Island dgtsed contours. The SRTM-DEM adjustment was then performed usng the altmetrc datasets for each sland n order to compute the respectve shfts. These values are presented on Table 4. Fgure 5. Fogo Island dgtsed contours Island Datum name Δλ ( ) Δϕ ( ) Boavsta Curral 15.2-3.7 Mao Monte Vermelho 33.1 9.1 Sal Monte Vermelho 11.6-1.0 Santago Boa Entrada 35.2 7.7 Brava Monte Fontanhas 33.1 10.3 Fogo Ponto Alto do Sul 33.6 9.6 Sto. Antão Gudo Cavalero 17.8-11.0 Sta. Luza Gudo Cavalero 17.5-11.1 São Ncolau Monte Bssau 15.3 12.7 São Vcente Base NW 16.5-10.6 882 4

Table 4. Datum shfts found for maps of 10 slands of Cape Verde archpelago. These shfts could be appled to each map georeferenced on the local system, turnng them nto the WGS84 system. There were no avalable data to evaluate the results besdes the Landsat ETM+ (from the GLCF datasets). The concdence of the maps wth the satellte magery s generally good and accordng to the postonal precson of those mages (around 40m as prevously mentoned). 3.3 East Tmor case study Three East Tmor 1:50 000 maps (1/4 degree), produced n the 60 s on UTM projecton (zone 51. south) were provded by Insttuto de Investgação Centífca Tropcal. Snce the three maps (Dl, Maubsse and Ermera) are adjacent, smlar dfferences from local datum to WGS84 are expected. Those maps were dgtsed and georeferenced usng grd ponts and, as the prevous cases, some ponts on the contour lnes were vectorzed. For a total of sx areas, two from each map, the results obtaned durng the adjustment are shown n Table 5. Map sheet Locaton Δλ ( ) Δϕ ( ) Ermera 1-5.5 28.1 2-4.5 28.1 Maubsse 1-5.5 28.2 2-5.3 27.4 Dl 1-5.5 26.9 2-5.1 26.9 Mean -5.2 27.6 Std. Dev. 0.4 0.6 Table 5. Datum shfts found for 3 maps of East Tmor. The computed standard devatons (equvalent to about 12 and 18 meters) confrms the possblty to georeference these maps nto WGS84 wth a precson adequate to ther scale. 4. CONCLUSION The method to adjust the altmetrc data ndcated a 5 meters planmetrc precson for the SRTM models. Takng the SRTM DEM as reference ths technque provdes an adjustment of other data wth a 5 meters precson, whch means 1/20 of the SRTM resoluton. Ths allowed the georeferencaton of old cartography nto WGS84 wthout any feld work, wth varous types of geodetc data, mantanng a rgour generally accepted for the map scale. The work nvolved on contour lnes dgtsaton s relatvely small, makng ths technque smple but effectve to use. The method has a potental to be appled n other stuatons, namely n the orentaton of optcal satellte mages wth stereo capabltes. A relatve DEM can be extracted and then correctly georeferenced to WGS84, wthout the nterventon of ground control. REFERENCES Rabus, B., M. Eneder, A. Roth, R., Bambler, 2003. The Shuttle Radar Topography Msson a new class of dgtal elevaton models acqured by spaceborne radar. ISPRS Journal of Photogrammetry and Remote Sensng. (57), p. 241-262. Rodrguez, E., C.S. Morrs, J.E. Belz, 2006, A global assessment of the SRTM performance, Photogramm. Eng. Rem. Sens., Vol. 72, p. 249-260. Smth, B., D. Sandwell, 2003. Accuracy and Resoluton of shuttle radar topography msson data. Geophyscal Research Letters, Vol. 30, No. 9. IGP, 2007. Web pages of the Insttuto Geográfco Português : http://www.geo.pt, (accessed on Aprl 2008) Schenk, 1999. Dgtal Photogrammetry Vol. I. Eneder, M., 2005. Interferometrc DEM Reconstructon of Alpne Areas Experences wth SRTM Data and Improved Strateges for Future Mssons. Proceedngs of the EARSel Workshop on 3D Remote Sensng. Porto, June, 2005 (CD- ROM). Gonçalves, J., Fernandes, 2005, J. Assessment of SRTM-3 DEM n Portugal wth topographc map data. Proceedngs of the EARSel Workshop on 3D Remote Sensng. Porto, June, 2005. (CD-ROM). Jacobsen, C., 2005. Analyss of SRTM elevaton models. Proceedngs of the EARSel Workshop on 3D Remote Sensng. Porto, June, 2005. (CD-ROM). ACKNOWLEDGEMENTS To Insttuto de Investgação Centífca Tropcal and Centro de Investgação em Bodversdade for provdng the cartography used by ths study. To Insttuto Geográfco Português for the altmetrc nformaton. SRTM provded by NASA/NGA/USGS. 883 5

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