If you missed my posts on terrain derivatives or SAGA, you may want to jump back and read those first. Today I’m going to talk about two very common terrain derivatives and how to calculate them in SAGA. I’m going to keep going with the same DEM from Mt. Gilead State Park I used in the Intro to SAGA post and loaded the raster the same way as before. Again, if you need a refresher on importing data or how the grid system works, go back to the Intro to SAGA post.
There’s a really simple way to simultaneously create 16 terrain derivatives in SAGA from a DEM. Go to the “Geoprocessing” tab on the main menu and then go down to “Terrain Analysis.” The last option on that sub-menu should be “Basic Terrain Analysis.”
When you click on “Basic Terrain Analysis,” you’ll need to select your grid system and the elevation layer in the top two drop down menus. Click “Okay,” and that’s it!
SAGA will take anywhere from 2 to 30 min to process this depending on the extent and resolution of your DEM. Once it’s done, your data panel will display your new terrain derivatives.
Double click the layer name, and you can view your new data! If you want to export your data to a permanent file or another GIS software, just go to “Geoprocessing” on the main menu and scroll down to “File.” From there, select which type of file you want to work with (grid for most things, or shapes for the channels and drainage basins). Then click export and the file type you want to export as. I usually use GeoTiffs to export grids and shapefiles to export shapes. When you click the option for the format you’re exporting the file as, there will be a popup menu really similar to the Basic Terrain Analysis popup– select the data you want to export and name the new file, and you’re done!
Of this long list of data the Basic Terrain Analysis tool creates, the ones I use most often are slope, aspect, and topographic wetness index. In other sections of the terrain analysis menu you can get calculate derivatives individually– I’ll go through and define some of the weirder ones and give you some ideas about how to use them in a future post.
Between this post and the Intro post, you should have enough information to get your own projects started in SAGA. If you run into any confusion, comment away and I’ll see if I can help!