The digital surface map (DSM) is often used interchangebly with digital elevation models (DEM). In the end, what we're trying to convey is that that file you're looking at contains information about the elevation of the stitched mosaic. To best get to the information in these files, you'll want some sort of GIS software that can handle the data, in this walkthrough, I'll be using QGIS. QGIS is a free and open source software for manipulating geospatial data ( http://www.qgis.org/ ).
To begin, you'll want to download the Cropped DSM from your downloads page. This can be found under the professional service results. Select the "Tif" with the description of "Cropped Digital Surface Map" from the drop down as seen in the image below. The image you download should be blank, see this article for why:
You will need to have specifically requested this as an output either via a "New Export" or on upload. You must also have a pro subscription. Our DSMs are in beta due to the fact that we have no way for users to input ground control points to increase the accuracy. You may notice bowing with certain surface maps, this is the nature of stitching images together and using that to create a surface map.
Now I'll go through a workflow to help identify some lower elevation locations in the data.
- In QGIS, start a new project (File -> New Project)
- Click the Checkboard icon with a plus to add a new raster layer, you can also do this via the menu Layer -> Add Layer -> Add Raster Layer. Or, my preference, open the folder that contains the image and drag and drop it onto the white canvas in QGIS. Regardless, this is the button I'm referring to:
- You should now see a black and grey image rendered on the map:
On the left hand side, under the "Layers Panel", you'll now see the raster layer loaded along with the legend for the colors. Our DSMs at the time of writing this are relative elevations in meters. In this case, black will represent a relative elevation of 291.5 meters and white will represent 306.993 meters, with the ramp going from black to white.
At this point, you may want to load in the mosaic so that you can see visually what's going on at various elevations (you just follow the same steps as above). Now lets say I want to better visualize what's going on with the data in this case, since black to white is not very friendly to a human, we'll change it to something more useful. The workflow is very similar to this article so I won't be going in as much depth:
- Right click on the layer, select properties, you'll now see something similar to this:
- Select "Style" from the left hand side menu (1)
- Under the "Band rendering" group, change the "Render Type" to "Singleband psuedocolor" (2) and leave the band as "Band 1 (Gray)" (3) or change it to that value if it didn't default to it.
- For me I'll be loading "PuOr" (4) for the color, setting the mode to "Continuous" (5), but you can do whatever you want (also see the article I linked to see what everything means)
- Click "Classify" (6) under the table
- Finally click "Apply" (8)
- You should now see your image rendered like the below:
The legend has been updated to better understand what is going on with our data and we can now see variation in elevation more clearly. You may find it useful to mess with the min/max values in the style dialog.
We now have a pretty picture, which you can export following the steps outlined in this article under the "Exporting the Rendered Geotif" section towards the bottom. But let's say I want to know the elevation between these two points on the image:
I've overlayed the original mosaic on top of the DSM so that I can better identify the variations. But to identify the elevation at those two points:
- Activate the identify tool in one of the following ways:
- From the top menu select "View" -> "Identify Features"
- From the menu of buttons, look for the cursor with an "i" in a blue circle:
- Use the keyboard shortcut Ctrl+Shift+I
- Right click on the point on the map that is of interest and click "Identify All" or the layer of interest
- You should now see an "i" on you cursor when hovering over the map. From the left hand side, be sure you click the DSM to make it the active layer. You'll know which is the active layer by the fact that it will be underlined.
- I'll start by clicking on the (1) from the above image which results in an "Identify Results" panel appearing that displays the information about the point I clicked. My window was too small at the time, so I used the drag feature of the panel to expand it:
You can see from my image that I actually have the RGB mosaic as the top most layer, but because I made the active layer the DSM, I can get the data values from that layer. There is also an option to display the data from all layers when you click.
- I now have the data visible for that point that I select, which is at 302.956 meters and I also get the X, Y coordinates which are represented as longitude and latitude respectively.
- I repeat this step again and get that the other point is at 295.352556. And you can repeat this step as many times as you'd like.
This tutorial from QGIS might also be helpful in providing useful analyses of your data: http://www.qgistutorials.com/en/docs/sampling_raster_data.html
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