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Effect of the SRTM global DEM on the determination of a high-resolution geoid model: a case study in Iran

Effect of the SRTM global DEM on the determination of a high-resolution geoid model: a case study in Iran
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  J Geod (2005) 79: 540–551DOI 10.1007/s00190-005-0006-8 ORIGINAL ARTICLER. Kiamehr  ·  L.E. Sj¨oberg Effect of the SRTM global DEM on the determination of ahigh-resolution geoid model: a case study in Iran Received: 19 January 2005 /Accepted: 8August 2005 / Published online: 10 November 2005© Springer-Verlag 2005 Abstract  Anyerrorsindigitalelevationmodels(DEMs)willintroduceerrorsdirectlyingravityanomaliesandgeoidmod-els when used in interpolating Bouguer gravity anomalies.Errors are also propagated into the geoid model by the topo-graphic and downward continuation (DWC) corrections intheapplicationofStokes’sformula.Theeffectsoftheseerrorsare assessed by the evaluation of the absolute accuracy of nine independent DEMs for the Iran region. It is shown thatthe improvement in using the high-resolution Shuttle RadarTopography Mission (SRTM) data versus previously avail-able DEMs in gridding of gravity anomalies, terrain correc-tions and DWC effects for the geoid model are significant.Based on the Iranian GPS/levelling network data, we esti-mate the absolute vertical accuracy of the SRTM in Iran tobe6.5m,whichismuchbetterthantheestimatedglobalaccu-racyoftheSRTM(say16m).Hence,thisDEMhasacompa-rable accuracy to a current photogrammetric high-resolutionDEM of Iran under development. We also found very largedifferences between the GLOBE and SRTM models on therangeof  − 750to550m.Thisdifferencecausesanerrorintherange of   − 160 to 140mGal in interpolating surface gravityanomalies and  − 60 to 60mGal in simple Bouguer anomalycorrectionterms.Intheviewofgeoidheights,wefoundlargedifferencesbetweentheuseofGLOBEandSRTMDEMs,inthe range of   − 1.1 to 1m for the study area. The terrain cor-rection of the geoid model at selected GPS/levelling pointsonly differs by 3cm for these two DEMs. Keywords  Digital elevation model (DEM)  ·  Geoid  ·  Iran  · Shuttle Radar Topography Mission (SRTM) R. Kiamehr  ·  L.E. Sj¨obergDivision of Geodesy, Royal Institute of Technology,SE -100 44 Stockholm, SwedenE-mail: ramin@infra.kth.seE-mail: sjoberg@geomatics.kth.seTel.: +46-8-7908603; +46-8-7907330Fax: +46-8-7907343 1 Introduction Adigitalelevationmodel(DEM)isacomputerrepresentationof the Earth’s surface; it provides a base dataset from whichtopographic parameters can be digitally generated. DEMsare used, e.g., to determine the terrain correction and down-ward continuation (DWC) corrections in geoid modelling,geo-morphological simulation and classification and hydro-logical run-off modelling.However, a DEM is only a  model  of the elevation sur-face,andlikeothermodels,itissubjecttoerrors(e.g.,Hiltonet al. 2003). Like any other source of data in geoid determi-nation (e.g., global geopotential models and gravity data), itis important to evaluate the accuracy of the DEM in the areaof interest before using it. The accuracy of DEMs usually isnot uniform because they use various data sources in theirconstruction. To make such an assessment, users must firstbe aware of the impact of errors of the DEM (e.g., Merry2003).The error can be directly estimated by comparing theheights extracted from a DEM and their values interpolatedfrom GPS/levelling data (where GPS is used for the hori-zontal control). However, because of the presence of datumproblems among different types of heights, in particular inthe determination of heights by levelling, this effect must beeliminated before any detailed discussion.There have been a few studies on DEM-error effects ingeoid modelling. Merry (2003) compared the effect of someregional and global DEMs according to Molodensky’s the-ory for computing the G1 term and the influence of the gridsize of DEMs on quasi-geoid in a small part of SouthAfrica.Based on his work, an error in height of 120m introduces aRMSerrorintheheightanomaly( ζ  )andG1termintheorderof 7 and 2cm, respectively.After the release of the high-res-olution Shuttle Radar Topography Mission (SRTM) DEM,it is of interest to investigate the quality and effect of thisnew data source versus regional and global DEMs in geoidmodelling.The test area that we have chosen for this purpose is inIran,boundedby25 ◦ N < ϕ <  40 ◦ N   and44 ◦ E < λ <  64 ◦ E
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