How to plot the DEM (Digital Elevation Model) of any region in Matlab

GIS and Remote Sensing is the part and parcel of Civil Engineering. Water resources engineering, hydrology, advanced surveying, etc. are some of the branches of civil engineering which make extensive use of GIS technology. A majorly used data for the study associated with these branches is the DEM (Digital Elevation Model) or DSM (Digital Surface Model) data. In the present article, we will see what is DEM and how we can plot DEM data in Matlab. I have illustrated the plotting of a sample location of the Badrinath City in Uttarakhand, India for a better understanding.

Google earth image of the area, whose DEM data plotting in Matlab is illustrated. 

A digital elevation model data comprises of elevations of a desired set of geographical points. There are various sources to extract the DEM data of a region. One of the most widely used digital elevation models is given by the SRTM (Shuttle Radar Topography Mission), an international project to acquire radar data, used to create the first near-global set of land elevations. It is available for free at the famous United States Geological Survey's EarthExplorer website: http://earthexplorer.usgs.gov/. The website provides various versions of the data including the SRTM Non-Void Filled,  SRTM Void Filled, SRTM 1 Arc-Second Global types of dataset available for download in the DTED, BIL and GeoTIFF file formats. Click here to know about each type of available SRTM dataset in detail.

I recommend downloading the SRTM 1 Arc-Second Global dataset in the GeoTIFF format for general and research studies.This type of DEM data offers worldwide coverage of void filled data at a resolution of 1 arc-second (30 meters). This means that the dataset is freely available for any region world-wide at a spatial resolution of 30m. The GeoTIFF format means that the DEM data is in raster (TIFF format) format with embedded geographic information i.e. the dem itself is a 3d plot, with x-y plane representing the geographical coordinates and z-axis representing the associated elevation value of the coordinate. In simpler terms, it is a normal b&w (might vary with type of view) image with packed elevation and geographical coordinates information.

Figure 1: Screenshot of the US Government Geological Survey's Earthexplorer website, captured while selecting the coordinates-bound region whose DEM data is to be downloaded

The dem data after being downloaded can then be viewed in any picture viewer as a raster image or can be opened with a GIS software like the ArcGIS, QGIS, Microdem, etc. You can also crop the raster data in order to restrict it only to your needed set of coordinates using these softwares or process it according to your need. As already said, the downloaded data is in raster format, so to plot it in Matlab, we need to convert it into ASCII (basically, text format) format. This can be easily achieved using the built in "From raster" conversion tool of ArcGIS software, which gives you an option to convert the raster dem file that you have opened in ArcGIS into ASCII format and export it as a text file.

Figure 2: A sample SRTM DEM data of a region in Uttarakhand, India, showing a b&w image.The brightest regions depict the highest elevation values and the darkest regions depict the lowest elevation values.

Figure 3: The raster SRTM DEM data shown in figure 2, cropped to the desired coordinates-bound (bounded by 79⁰29’24’’ E- 79⁰29’42’’E Latitude and 30⁰44’42’’N - 30⁰44’24’’N Longitude) region in ArcGIS

The dem data in ASCII format is basically a text file consisting of an array of elevation values for the respective geographical coordinates of the given area. The range of geographical coordinates(lattitudes and longitudes) depends on the coordinates of the edges of the desired region to be plotted (for example, in the present case(fig. 3) it is 79.49003 to 79.4950 degrees E Lattitudes and 30.74003 to 30.7450 N Longitudes). Now, as the spatial resolution of the data is 1 second, i.e. 1/3600 degrees, take the least count of the coordinates as 1/3600 degrees. The ASCII dem text file is opened in notepad to get the elevation values corresponding to the respective coordinates.

Finally, the code to be written in Matlab is shown below:

X= 79.49003:1/3600:79.4950;
Y= 30.74003:1/3600:30.7450;
Z= [
 3126 3119 3112 3106 3097 3092 3093 3100 3104 3106 3106 3109 3114 3119 3123 3126 3128 3128;
 3123 3116 3111 3105 3096 3089 3093 3099 3106 3108 3108 3110 3116 3120 3123 3126 3128 3129;
 3124 3115 3108 3103 3093 3087 3091 3099 3106 3109 3109 3113 3117 3121 3126 3128 3129 3129;
 3123 3114 3105 3097 3088 3086 3093 3100 3109 3112 3114 3116 3119 3123 3127 3128 3128 3128;
 3119 3112 3102 3091 3084 3086 3095 3102 3109 3115 3116 3118 3121 3124 3127 3127 3126 3126;
 3112 3106 3100 3092 3086 3091 3096 3103 3110 3115 3118 3120 3123 3125 3125 3124 3124 3126;
 3107 3103 3099 3094 3092 3093 3098 3105 3113 3117 3120 3124 3125 3125 3124 3123 3124 3128;
 3108 3103 3098 3093 3091 3091 3098 3105 3113 3118 3122 3124 3125 3124 3124 3125 3126 3129;
 3109 3101 3095 3092 3084 3086 3094 3101 3110 3117 3121 3123 3123 3123 3125 3127 3128 3127;
 3112 3100 3095 3091 3083 3080 3088 3095 3102 3111 3116 3119 3119 3121 3123 3127 3126 3124;
 3117 3103 3095 3089 3081 3079 3083 3089 3095 3102 3107 3115 3117 3119 3121 3123 3124 3123;
 3114 3104 3096 3090 3083 3079 3081 3085 3089 3093 3099 3107 3112 3117 3118 3120 3121 3122;
 3111 3107 3099 3094 3088 3084 3082 3084 3083 3086 3092 3099 3105 3110 3115 3116 3119 3121;
 3107 3104 3097 3095 3092 3089 3088 3082 3081 3084 3088 3095 3101 3104 3109 3114 3118 3120;
 3102 3098 3095 3096 3095 3092 3089 3081 3079 3082 3085 3091 3097 3102 3107 3111 3113 3115;
 3107 3101 3098 3100 3100 3095 3089 3082 3079 3080 3084 3086 3092 3099 3103 3106 3108 3110;
 3115 3110 3108 3106 3103 3095 3089 3083 3078 3079 3082 3086 3092 3098 3101 3106 3110 3109;
 3114 3112 3111 3108 3105 3099 3092 3084 3078 3077 3081 3086 3093 3096 3102 3108 3112 3112;];

surf(X,Y,Z);
xlabel('EAST(Decimal Deg.)');
ylabel('NORTH(Decimal Deg.)');
zlabel('Elevation(m)');
axis tight
colorbar

The 2d matrix Z represents the set of elevations( in meters), copied from the dem ASCII text file, whereas the row-matrices X and Y represent the coordinate axes for the given region. Note the dimensions of the row matrices X (18, here) and Y (18, here), and that of Z (18*18, here).

Figure 4: Screenshot of the above code being run in Matlab to plot the SRTM DEM

The code is run in Matlab, and the extracted plot can then be exported and saved as a jpeg image, as shown in Figure 5.

Figure 5: Final 3-D plot of the SRTM DEM data of the desired region in Matlab, with the X-Y plane showing the geographical coordinates of a point in the region, and the Z-axis showing the elevation of the respective point.

From the SRTM Digital elevation data that was extracted for the region of interest, it is evident that the selected region consists of undulating topography. As we have seen, the downloaded SRTM DEM data consists of elevations of square grid cells of 30m x 30m size (30m spatial resolution). This makes it difficult to visualise the slope of points in the study area, within each  cell. This problem, as we can see, has been very well solved by Matlab; and inspite of resolution as low as 30m and the region being as small as 0.3 sq. km, we can clearly picturise the topography of the region and use this plot for further estimations. 

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