Transforming SRTM data to WAsP maps

SRTM coordinates are non-projected (latitude, longitude). Horizontal reference system (datum) is WGS84 and vertical reference is the EGM96 geoid. Transforming SRTM data to WAsP elevation maps therefore require the following:

• Transformation of geo. coordinates to a metric system
• Transformation of grid point elevations to height contours
• Transformation of WGS84 to another datum – if need be

One possible procedure is outlined in detail in Steps 1-6 below. The tools available in this example are assumed to be:

• Surfer 8
• WAsP Map Editor 9
• WAsP Utility Programs 3

Other procedures and software exist to load, view and transform SRTM data, including generation of vector height contour maps. Different WAsP users have mentioned:

• USGS dlgv32 Pro from the US Geological Survey
• Global Mapper from Global Mapper Software LLC
• SAGA – System for Automated Geoscientific Analyses from University of Göttingen

as alternatives. A note on the use of SAGA for establishing WAsP vector maps can be downloaded here.

Step 1: Download data from the SRTM site

1. Download SRTM data from the  SRTM site (http://dds.cr.usgs.gov/srtm/version2_1/)
2. Download tiles to cover the met. station or wind farm area out to approx. 10 km from any site

Note, that the file names refer to the lower left (LL) corner of each 1 by 1 degree tile. The SRTM HGT format is similar to the GTOPO30 format, which is supported by Surfer.

Step 2: Convert the HGT file to GRD format using Surfer

1. Unzip the downloaded ZIP file
2. Rename the HGT file to DEM
3. Create HDR and STX files (with the same file name)
4. Insert upper left corner coordinates (signed) in the HDR file
5. Start Surfer and choose Grid | Convert…
6. Open the *.HDR file
7. Save grid as *.GRD file (with the same file name)

The result is a Surfer GRD file in geographical coordinates (WGS84). Inspect the grid for voids (undefined values) and spikes and wells using Surfer. Remove spikes and wells by inserting a sensible elevation value using the Surfer grid editor.

Step 3: Change the coordinate system to UTM

First, convert the grid file to a list-of-points file in Surfer:

1. Choose Grid | Convert…
2. Open GRD file and save as ASCII XYZ (*.dat)
3. Repeat steps 1-2 for each tile, if need be

Second, transform the geographical coordinates to UTM coordinates. If two or more tiles are needed to cover the area, merge the DAT files into one DAT file. Now, you can use the Geo-Projection Transformer utility program to make this transformation, using File | Transform XYZ-file. Alternatively, in the WAsP Utility Programs Command Prompt (CP) window:

1. Write UTM 1 at the prompt
2. Enter the UTM zone number
3. Drag and drop the DAT file from Windows Explorer to the CP window
4. Write ‘y’ to ‘File of (X, Y, Z) coordinates’
5. Drag and drop the DAT file again
6. Change the file name extension to *.xyz

The result in both cases should be an ASCII XYZ file in metric map coordinates (WGS84).

Step 4: Make a metric GRD file

1. In Surfer, choose Grid | Data…
2. Open the XYZ file as ‘Golden Software Data’
3. Choose ‘Skip leading spaces’ and ‘Treat consecutive delimiters as one’
4. Choose a name for ‘Output Grid File’
5. Invoke Filter data... if you want exclude e.g. certain high z-values in the data
6. Set values for ‘Grid Line Geometry’, i.e. grid size and extents of modelling domain

The result is a Surfer GRD file in metric map coordinates (WGS84) covering the modelling domain. Surfer has made a complete grid without voids by interpolation (e.g. Kriging).

Step 5: Make a contour map in DXF format

1. Create a new contour map in Surfer, using the GRD file as input
2. Choose the appropriate contour levels in the Properties | Levels window
3. Export the height contours to a 3-D AutoCAD DXF file from the Map | Contour map... menu
4. Optionally, repeat steps 1-3, using different different Levels and Limits

The result is one or more maps in DXF format, with different map limits and contour line intervals. For a met. station site one might choose to make a 20 by 20 km² map with 50-m contours, a 10 by 10 km² map with 10-m contours and a 2 by 2 km² map with 5-m contours – all centered around the met. station. These maps are then merged in Step 6 below.

Step 6: Make a WAsP contour map

1. Open the DXF file in the WAsP Map Editor
2. Add and Replace... to merge several maps
3. Check the map contours for spikes and wells
4. Transform to any other datum, if need be
5. Compare to a scanned background map
6. Check vertical datums and compare elevations
7. Add spot heights and other details close to the site(s)
8. Add roughness change lines – including the coastline, if any
9. Save the map as WAsP ASCII map file (*.map)

Mission accomplished!

The result is a WAsP ASCII map that can used for WAsP analysis and/or application. The procedure described above is not the simplest possible, but it is reasonably fast and furthermore has a number of advantages:

1. The raw grid is inspected for voids, spikes and wells
2. The final map grid is (hopefully) complete, without voids, spikes and wells
3. Contour map(s) are made to specified levels and limits
4. The procedure steps can be adapted to the specific site(s)
5. The WAsP user gets to know the map data in detail...

Do remember...

The SRTM elevation data have not been tested in all parts of the world for WAsP modelling. Do remember, that when you carry out a WAsP project, you are in charge and responsible for evaluating the WAsP inputs and the results – including the availability and quality of SRTM data.

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