There are two parts to this step. In the first part, elevation, slope and aspect data are extracted from the rasters and associated with each point in the forest grid. This results in three separate point files. The second part involves consolidating the elevation, slope and aspect data into the main forest grid file, which already contains the point coordinates.
In QGIS, click on Manage and Install Plugins. Search for and install a plugin called Point sampling tool.
In the QGIS table of contents, check the boxes for the relevant layers only. Start with the SRTM elevation layer and the fine 0.01 degree grid over forest areas.
Under the Plugins menu, click on Analyses and Point sampling tool.
Use point sampling tool to add elevation to the grid file.
- Select the fine forest grid as the layer containing sampling points.
- Select the SRTM elevation data, band 1 raster.
- Specify the output location and file name. Check the box to load the resulting layer once the process is complete. Then click OK.
The resulting shapefile contains the elevation at each point.
Repeat the process for slope and aspect. There should be separate shapefiles for elevation, slope and aspect.
To consolidate the elevation, slope and aspect data with the main forest grid, click on the Vector menu and select Data Management Tools, then Join attributes by location.
Start by adding elevation data.
- Select the forest grid (with coordinates) for the target vector layer.
- Select the elevation points as the vector layer to join.
- Take attributes of the first located feature.
- Specify the output location and file name. Keep only matching records. Then click OK.
Repeat the process for slope and aspect data, but remember to change the target vector file to the most recent join.
The intermediary elevation, slope and aspect point files and the first two spatial joins are temporary files that can be deleted once the final shapefile is produced. The final join should have a clear file name consistent with the other Collect Earth survey files (no spaces, alpha-numeric characters only).\
After consolidating elevation, slope and aspect data with the main forest grid, right-click on the layer and review the data attributes table to confirm that the process has been completed properly.