Comparative Analysis of Digital Elevation Models: a Case Study Around Madduleru River

Indian Journal of Geo Marine Sciences Vol. 46 (07), July 2017, pp. 1339-1351

Comparative analysis of digital elevation models: A case study around Madduleru River

Subbu Lakshmi. E1 & Kiran Yarrakula2*

Centre for Disaster Mitigation and Management, VIT University, Vellore, Pin 632014, India

Received 14 July 2016 ; revised 28 November 2016

High resolution DEM is generated from Cartosat-1 stereo data. The performance of different DEMs is evaluated based on error statistics. To identify the hill profiles, the TIN plots have generated and compared for SRTM, Cartosat -1, and SOI toposheet. The study divulges that, elevation values of Cartosat-1 DEM are better in flat terrain and SRTM images in hilly region produced better, when compared each other.

[Keywords: Cartosat-1 DEM, SRTM-DEM, Google earth, Survey of India Toposheet, Accuracy Assessment, Digital elevation model]

Introduction corresponding image points are identified in the Cartosat-1 DEM with 2.5m spatial particular stereo images In general, the accuracy resolution and vertical resolution 7.5m is intended of Cartosat-1 DEM seems to be fine in the flat to be used for generating DEM. The ground terrain which is helpful to interpret the land control points and geometric model are the features13. Past few years many scientists and essential components required for generating researchers have done a series of local and global DEM from stereo data1. DEM in variety of assessments of these elevation products. Many application such as land use land cover to analyze new technologies are giving opportunities for the spatio-temporal change on the river2&3, generating digital elevation models in remote cadastral mapping, to assess the vertical sensing to determine Earth surface elevation at characteristics of topographical variability of increasing resolution for larger areas14. DEMs are urban built-up landscape4&5and Hydrological very useful that reflect the importance of the modeling to obtain information about flow lines, availability of global, consistent, and high quality slope, watershed boundary, elevation, DEM. In this paper an attempt has been made to contour6&7&8and other GIS applications. Elevation examine the accuracy of DEM derived from data generated from the satellite imageries is Cartosat-1 DEM, SRTM DEM, Google earth and evaluated by cross checking the elevation values SOI toposheet for proper planning. Existing obtained from topographic maps9. Assessment satellite based DEMs still show large drawbacks can also be done by checking the elevation values with respect to consistency, availability, cost, of the contour generated using satellite images degree of resolution, and coverage. DEMs should with elevation data obtained from differential act as a carrier of geoinformation representing global positioning system (DGPS) and global terrain features associated to the earth surface. It positioning system (GPS) 10&11. The cost of DGPS should provide innovative mechanism for data is very expensive12, provides very good operational applications to carry out more issue accuracy of less than 1m. In the present study, due and analysis operations to investigate the complex to unavailability of DGPS data, the elevation interactions among geospatial features and values are feed using toposheet and the processes identified at the Earth surface15. Here 1340 INDIAN J. MAR. SCI., VOL. 46, NO. 07, JULY 2017 mainly, we are assessing the quality of Cartosat-1 data through comparison with three other data sets. To assess the accuracy of DEM, a random of 59 control points are selected from the study area and interpreted the elevation values at each point. In this study elevation data of Cartosat-1 DEM, SRTM DEM, Google earth and SOI toposheet values are compared for the region of Anantapur and Kadapa districts of Andhra Pradesh, India.

Materials and Methods The DEM comparison has been performed for the region of Anantapur and Kadapa districts, Andhrapradesh, India. Dorigallu and Gorivikanuma, Kokkarajukonda are the main hill areas covered by the dense forest extending from west to east in the study area. The height of this forest area varies from 372m to 755m. High resolution Cartosat-1 stereo data is used for DEM generation and it is compared with SRTM DEM with vertical resolution of 30m. Google earth and

SOI toposheet with 1:50,000 scales are also used Fig. 1 Geographic location of the study area for DEM comparison. Figure 1 shows geographic location of the study area. Table 1 shows product details of the images. Table1 Data resource description

S.NO Image Resolution Satellite Area Date of procurement used 1 PAN 2.5m IRS-1C (Cartosat-1) Anantapur and March 2010 Kadapa 2 SRTM 3-ARC Shuttle Radar Anantapur and September 2014 DEM seconds Kadapa 3 Google 0.15(Highest) Digital globe (Astrium, SPOT) Anantapur and January 2014 Earth Kadapa 4 SOI 1:50,000 Photogrammetric interpretation Anantapur and Procured in scale of aerial photography, LIDAR and Kadapa 2013.Updated for major toposheet other remotesensing techniques. details during 2014-2015

DEM is mainly used for representing the extraction of sensor information from RPC file is terrain surface in 3D form and to interpret the done to carry out the interior and exterior topographic features16. High resolution Cartosat-1 orientations18. Rational function is chosen as stereo kit with rational polynomial coefficient category of geometric model19. Stereo image pair (RPC) file is used to develop digital elevation is loaded which is in TIFF (Tagged image file model. Cartosat -1 ortho image is a panchromatic format) form. Then RPC coefficients are specified image with spatial resolution of 2.5m17. For DEM for the Band A and Band F images using the extraction, a block file is created for the image Survey of India Toposheet (SOI) 57J/3 by identity. The type of geometric model Cartosat-1 checking the minimum and maximum elevation RPC is assigned for the created blocks. The values of the study area. Pyramid layers for the LAKSHMI & YARRAKULA: COMPARATIVE ANALYSIS OF DIGITAL ELEVATION MODELS 1341 image in the block file will get activated of SRTM DEM, the contour is generated using 3D automatically. The pyramid layers are used to analyst option in ARC GIS software. By optimize image display during automatic tie point providing corresponding latitude and longitude collection and also in DEM extraction process. values of the chosen 59 points, the elevation Tie points are automatically generated using LPS values of the particular location is taken from the software and it act as a 3D reference point to SRTM contour, and these values are compared compute the quality of digital elevation model. To with contours generated from Cartosat -1 DEM. In establish the relationship between stereo images, case of toposheet, using latitude and longitude the sensor and ground in the block, the values of 59 chosen points, the corresponding triangulation process is done by providing ground elevation values are identified in toposheet and control points (GCPs) 20. GCP are the points on these values are then compared with Cartosat-1 the earth surface of known location (latitude, elevation values. Figure 2 shows the detailed longitude, elevation). Accurate ground control methodology for generating digital elevation points in the overlap area would results in uniform model using Cartosat-1 stereo data. DEM with high accuracy21. In the present work, the ground coordinates of GCPs are derived from Collection of CARTOSAT-1 stereo Collection of SRTM Identifying study area from image DEM Google earth satellite image of IRSP6 Resourcesat-II. Tie points are generated in the overlap area between Create block project (.blk) Generating contour for the Using selected 59 data

in LPS 9.0 software study area using ARCGIS points for the study area the two stereo images. Sometimes the ground software analyzing the elevation values control points are taken as check points for Set Geometric model as Cartosat RPC Generating contour using generating DEM. The points with known ground ARCGIS software positions are check points used for assessing the Set projection type and Datum Using selected 59 data 22 points for the study area accuracy . Elevation values are derived from analyzing the elevation Load stereo image pair values toposheet for the corresponding well defined GCP points. The block triangulation is performed after Tie point generation adding GCP and elevation values. The block Triangulation triangulation estimates the position of each image Adding GCP, check point (for absolute DEM) in a block at the time of image capturing and establishes the relationship between images within Block adjustment and 23 processing a block sensor model and the ground . The triangulation process is run to check the accuracy Generation of contour from Cartosat 1 DEM of GPCs and tie points. Then the block Ortho image generation adjustment is carried out that simultaneously process all the images within the block which Using selected 59 data points for the study area minimize the error. Finally the DEM is extracted analyzing the elevation values with DEM cell size of 10m, after checking the accuracy of GCP and tie points by using DEM Accuracy assessment of different DEM elevation extraction. Reference coordinate system, values projection type, vertical and horizontal datum are used as Geographic lat/long, UTM, and WGS84 respectively. Once the DEM is generated, Fig. 2 Detailed methodologies for the comparative analysis of automatically the contour can be extracted from various DEMs the DEM. Finally the ortho image is generated using the DEM derived from Cartosat-1 stereo DEM and ortho image generation using Cartosat image24 it will resample the triangulated images 1 stereo images and create orthoimages which are planimetrically Cartosat-1 satellite is the first high true images; represent the ground objects in their resolution optical along track stereo imaging real world X and Y position25. Then the Cartosat-1 system launched on 5th May 2005, built by ISRO contour is draped over Google earth to identify the (Indian space research organization) with base to study area and by giving corresponding latitude height ratio of 0.62 and covers a swath of and longitude values of the chosen 59 points the 30km26&27with 2.5m spatial resolution in the elevation values of the particular location. In case 1342 INDIAN J. MAR. SCI., VOL. 46, NO. 07, JULY 2017 visible region of electromagnetic spectrum the ground coordinates of GCP’s are marked in produce high quality DEMs for any location on BAND-A and BAND-F images using point the earth surface virtually28&29. The study area measurement tool . The elevation data for the covering longitude 78° to 78.34° E and Latitude corresponding locations for the BAND-A and 14.26° to 14.56° N with path number and BAND-F images are taken from the SOI row number are 546 and 329 respectively. The toposheet. Tie point is generated in the overlap sensor is PAN (Panchromatic). To generate DEM pare between the two stereo images. Here less from Cartosat-1 stereo data the required data sets number of GCP’s is given as a control points on include LPS (Leica photogrammetric suite) four corners and one at the centre of stereo image. software, Resourcesat II satellite image with Total of 13 GCPs are marked as control points and LISS-IV Mx sensor, SOI toposheet number 57 J/3 30 points are marked as tie points. Tie points are with 1:50,000 scale. The detailed information of generated automatically but that doesn’t seem stereo data is provided in Table 2. BAND-A and accurate. It produces some GCP mismatching BAND-F are image ﬁles, and supporting ﬁles error. Triangulation is carried out after feeding include “.aux, .txt, and .rrd” are procured from accurate controls points and elevation data, in the NRSA (National Remote Sensing Agency), corresponding stereo images. The distribution of Hyderabad. the control points and tie points are shown in Figure 3. Table 2 Detail information of the stereo pair

Stereo pair

Image 1 Image 2 Format

BANDA BANDF TIFF

BANDA_RPC BANDF_RPC .txt

BANDA BANDF .rrd

BANDA BANDF .aux

BANDA_MET BANDF_MET .txt

BANDA_RPC_ORG BANDF_RPC_ORG .txt

Rational polynomial coefficient concept Fig. 3 Distribution of control points (triangles), and tie points has been introduced by IKONOS30 represent (squares) over the stereo pair the relationship between the images and objects space and are terrain independent31 are specified Results and Discussion for the stereo image files by analyzing the Figure 5 depicts the DEM and minimum maximum elevation of the chosen study orthorectiﬁed image developed from Cartosat-1 area using toposheet. Due to unavailability of stereo data. From Figure 5 a) it is observed that DGPS data, the ground control points and maximum and minimum elevation of the ground elevation data are collected manually from is 110 to 900m respectively. Here bluish magenta RESOURCESAT II and SOI toposheet 57J/3 colour indicates the hilly area while the rest of the respectively. The well defined features like colour indicates low lying area. Figure 5 b) clearly permanent immovable features, road intersection, shows different features present in the study area and survey bench mark32 are chosen as GCP’s. To namely water bodies, village roads, national identify the location accuracy latitude, longitude, highways, railway lines, Madduleru River and its are taken from Resourcesat-II (LISS-IV Mx) and tributaries, reservoir, tanks, settlements, fields and other landscape features. LAKSHMI & YARRAKULA: COMPARATIVE ANALYSIS OF DIGITAL ELEVATION MODELS 1343

Figure 4 illustrate the refinement To identify the spatial accuracy of summary of triangulation process. From the Cartosat-1 DEM and SRTM DEM, randomly 59 refinement summary it is observed that RMSE points are identified in the study area with error for the stereo image seems to be 7.37 pixels. reference to SOI toposheet. After the development of DEM the ortho image is generated from the Cartosat-1 DEM which reduces geometric errors inherent within the imagery. The accuracy of orthorectified image depends on DEM and quality of the sensor model.

Image matching should be done accurately while marking the check points, GCP’s and tie points in the stereo image otherwise the RMSE error will increase. After triangulation, the Cartosat-1 DEM is generated with 10m cell size. The contours are also generated at 10m interval. The accuracy of DEM generated from Cartosat-1 stereo data could be improved with using good 33 distribution of GCP's . Figure 5 shows the Cartosat 1 DEM and ortho image covering the Fig. 4 Refinement summary of triangulation results parts of Anantapur and Kadapa districts.

Fig. 5 a) Cartosat-1 DEM at 10m resolution b) Ortho image (2.5m) generated from Cartosat-1 DEM

Figure 6 shows distribution of randomly selected estimation of topographical surface than SRTM points in the study area. It is observed that spatial DEM. Figure 7 comparison of elevation values of accuracy of SRTM DEM and Cartosat-1 DEM is Cartosat 1, SRTM and Google Earth with SOI extremely high when compared to toposheet toposheet. Figure 8 shows scatter plot of Cartosat- latitude and longitude values. Error statistics is 1, SRTM and Google Earth with respect to also performed for Cartosat-1, SRTM, Google toposheet. From both the Figure 7 and Figure 8, it Earth and toposheet. Table 3 shows estimated is confined that elevation estimated from Cartosat- error statistics of Cartosat-1, SRTM, and Google 1 DEM are found more close to toposheet when Earth with respect to SOI toposheet. The results compared with SRTM DEM. shows that Cartosat-1 DEM provide better

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Fig. 6 Distribution of randomly selected points in the study area

Table 3 Error Statistics of Cartosat-1, Google Earth and SRTM

Cartosat-1 Google Earth SRTM

Mean Error -5.28814 -8.84746 -5.88136

Mean Absolute Error 9.338983 11.08475 10.40678

Standard Error 1.620307 2.134273 1.923293

RMSE 40.61894 67.95861 45.17555

R2 0.994 0.993 0.993

Fig. 7 Comparison of elevation values of Cartosat 1, SRTM and Google Earth with toposheet LAKSHMI & YARRAKULA: COMPARATIVE ANALYSIS OF DIGITAL ELEVATION MODELS 1345

790 Cartosat Elevation Google Earth 690 Elevation SRTM Elevation

590

490 Elevation(m)

390

290

190 200 300 400 500 600 700 800 Distance (m)

Fig. 8 Scatter plot Cartosat-1, SRTM and Google Earth w.r.to toposheet

It is found that elevation obtained from Digital elevation model gives down-to- Google earth is not accurate when compared with earth information in various application fields36. other DEMs. Google earth facilitate mapping of The main advantage of Cartosat-1 mission is earth surface enabling 3D view of whole earth24. generation of DEM for production of ortho image The Google earth finds good for an indication of and visualization of terrain in 3D form at large height or slope and aspect of terrain features, but scale. The 2.5m radiometric resolutions of it is not sufficient for detailed design of any sort. Cartosat-1 sensors allow discriminating the It is because of the data has some errors in the objects, which reinforces the cartographic actual heights captured; the coarseness of data and potential of the sensor. Cartosat-1 images are the fact is the missing portions of data are appropriate in the following cases where DEM’S interpolated. But in flat area the quality of Google are required as a necessary form of input. earth elevation values is similar to SRTM DEM DEM is mainly used for creating contour which approximately produces 30m resolution of maps. Contour maps are derived from DEMs. data35. From this study it is clear that due to Using a series of mass points; contour lines for a unavailability DGPS and RPC’s in the hilly given range in elevation can be automatically region, the Cartosat-1 DEM is less accurate in extracted. Figure 9 shows the contour developed those regions. Visual comparison reveals that from Cartosat- DEM. Using latitude and longitude Cartosat DEM is performing better than the values, difference between the elevation values SRTM DEMs and Google earth. are analyzed with respect to toposheet37. Figure 10 shows the digital elevation model and 10m contours for SRTM DEM.

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Fig. 9 Digital elevation model and 10m contours Cartosat-1

Fig. 10 Digital elevation model and 10m contours SRTM

The efficiency of Cartosat-1, SRTM maximum and minimum elevation of the hill DEM, Toposheet TIN are carried out by digitizing ranges from 420 m to 600 m, the volume is 104.3 the hilly regions in Cartosat-1 DEM, SRTM km3 and the area is 1.44 km2. The maximum and DEM, SOI toposheet by generating TIN plot for minimum elevation of hill region in toposheet the Dorigallu forest area. The elevation difference ranges from 480 m to 620 m, the volume is 101.4 between the Cartosat-1 DEM, SRTM DEM and km3 and the area is 1.35 km2. Figure 12 shows the toposheet are analyzed from the generating results comparative hill profile of Cartosat-1, SRTM and and their corresponding hill volume, area are also toposheet. It can be seen that the volume of hilly estimated. Figure11 (a) shows the TIN plots of region of SRTM DEM shows maximum elevation Cartosat-1, Figure11 (b) shows the TIN plots of that are closely align with toposheet. From these SRTM and Figure11 (c) shows the TIN plots of results it is clear that due to unavailability of toposheet. The maximum and minimum DGPS, Cartosat-1 elevation values are not elevations of hill region in Cartosat-1 DEM range accurate in hilly areas. Table 4 shows hill profile from 420m to 620m and the volume is 119.0 km3 information Volume and Area comparison and the area is 1.45 km2. In SRTM DEM the between Cartosat-1, SRTM, and Toposheet.

LAKSHMI & YARRAKULA: COMPARATIVE ANALYSIS OF DIGITAL ELEVATION MODELS 1347

Table 4 Dorigallu hill profile information between Cartosat-1, SRTM, and Toposheet

S.NO Measurements Cartosat-1 SRTM Toposheet

1 Area (km2) 1.45 1.44 1.35

2 Volume(km3) 119.0 104.3 101.4

3 Minimum elevation(m) 420 420 480

4 Maximum elevation(m) 620 600 620

Fig. 11 (a) Extraction of hill profile from TIN plot of Cartosat-1

Fig. 11 (b) Extraction of hill profile from TIN plot of SRTM 1348 INDIAN J. MAR. SCI., VOL. 46, NO. 07, JULY 2017

Fig. 11 (c) Extraction of hill profile from TIN plot of Toposheet

Hill profile plot SRTM 650 CARTOSAT- 1 600

550 Elevation 500

450 0 100 200 300 400 500 600

Distance

Fig. 12 Comparative hill profile of SRTM, Cartosat-1 and Toposheet

To identify the profiles of rivers and tributaries, Madduleru river profiles of SOI toposheet, Madduleru river located in Anantapur and Kadapa Cartosat-1 DEM, and SRTM DEM. Table 5 shows districts of Andhrapradesh, is selected for the the stream order for the river profiles of Cartosat- analysis. Flow accumulation from both SRTM 1 DEM, SRTM and Toposheet. Red colour DEM and Cartosat-1 DEM are generated using indicates Cartosat-1 river profiles and tributaries, ARCGIS software and the stream order is sky blue colour indicates SRTM river profiles and generated using flow accumulation for both the tributaries, and brown colour indicates Toposheet DEMs. To compare the river profiles of Cartosat- river profiles and tributaries of Madduleru river. 1 DME and SRTM DEM, the reference river From the table it is identified that the total number profile is digitized using SOI toposheet. The of streams are 46111 in Cartosat-1. The total stream order is also provided using STRAHLER numbers of 8th order streams are 159 and 43 in method. Figure 13 shows the comparative river Cartosat-1 and SRTM respectively. Interestingly profiles and its tributaries of Madduleru river the total numbers of streams in SOI toposheet are generated for Cartosat-1 DEM, SRTM DEM and identified as 154 only. The reason behind it may Toposheet. Figure 13 shows the overlaid be due to improper updation of toposheets.

LAKSHMI & YARRAKULA: COMPARATIVE ANALYSIS OF DIGITAL ELEVATION MODELS 1349

Table 5 Generated stream order for the river profiles of Cartosat-1, SRTM DEM and Toposheet

S.NO Stream order Cartosat-1 SRTM Toposheet

1 1 26184 12693 74

2 2 10869 4886 45

3 3 4567 2236 25

4 4 2330 966 10

5 5 1074 647 -

6 6 643 325 -

7 7 285 76 -

8 8 159 43 -

Fig. 13 Comparative river profiles and tributaries of Madduleru river for Cartosat-1, SRTM and Toposheet

Conclusion information on topographic related analyses We compared the accuracy of digital especially in flat terrain region. Moreover, SRTM- elevation model (DEM) from high resolution DEM provided good elevation in hilly region. For Cartosat-1 stereo data with elevation values from this study, DGPS elevation values are not used SRTM (shuttle radar topography) DEM, Survey of due to high cost and unavailability. This study is India toposheet (SOI) and Google Earth. It is useful for environmental mapping tasks like observed that, an elevation value of Cartosat-1 avalanche hazard mapping, 3D perspective terrain DEM is better than SRTM, and Google Earth. The visualization, landform studies and topographic Cartosat-1 DEM provided good and satisfactory maps updating.

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