Creating Contours using ASTER DEM and Global Mapper

If none of the options on our Elevation Data page meet your needs, this help guide will show you how to:

Find a digital elevation model (DEM) for your area on the NASA Earthdata website
- We recommend the 1:50,000 / 30m CDEMs from Natural Resources Canada for locations within Canada, ASTER for non-Canadian locations
- If you're working in Ottawa, you have a couple of options: the 2012-2015 1m LiDAR-derived digital surface model (includes buildings/trees) or 2001 25cm DEM
Load a DEM into Global Mapper find your location
Create contours from the DEM in Global Mapper
Export the contours to AutoCAD Drawing (.dwg) or shapefile (.shp) format

**To create contours from a DEM in QGIS, use the Raster > Extraction > Contour built-in tool then save contours as shapefile or DWG.

**To create contours from a DEM in ArcGIS Pro, find details here.

ASTER is a technology that was used to create digital elevation models (DEMs) for the entire globe at a 30 metre resolution, which means each pixel is 30m x 30m and has one elevation value assigned to it. Global Mapper is a viewer capable of displaying elevation datasets and allows users to create contour lines from DEMs. If you have a site outside Canada for which you need elevation contours, this is how we recommend obtaining them.

While users can download a free trial version of Global Mapper at http://www.globalmapper.com/, this version is not the full version and does have considerable restrictions that do not allow the user to export to any file type. The Global Mapper found on the library's GIS Workstations has full functionality.

You can always contact us at gis@carleton.ca with questions.

1. Find a digital elevation model (DEM) in ASTER GDEM

**For Canadian locations, we recommend the CDEMs from Natural Resources Canada
Go to NASA Earthdata.
- If you do not have a login, it is quickest to register (it's free) at this point as you'll receive an email confirming registration. Then login.
Click the Find Data button near the middle of the webpage, or in the menu bar go to Data > Earthdata Search
In the search bar, search for 'aster'.
In the results, click the ASTER Global Digital Elevation Model V003 result. The page should look like this:
Click the grey down arrow beside the Back to Collections link to make the map pane larger.
Zoom into your area of interest.
When you are zoomed in to your location of choice, click the Search by spatial rectangle (icon is a square) button in the bottom right of the screen to highlight (in blue) the area surrounding your location. This will ensure that the search finds elevation tiles for the area surrounding your location as well.
ASTER GDEM tiles that intersect the rectangle you drew will appear. You can click on each tile to see a small preview.
Click on the grey up arrow beside the Back to Collections link. This will list the ASTER GDEM tiles that intersect your area of interest rectangle.
- If you just want a few of the available results, click on each tile you would like to download in the results pane then the Download Single Granule data button.
- If you want to download all the tiles (4 in this example), click the Download All button.
Once the tiles are downloaded, unzip it, then unzip the individual tile(s). You are now ready to make contours!

2. Loading your DEM into Global Mapper

When you first open Global Mapper, click on Online Sources.
In the Select Online Data Source to Download box, expand WORLDWIDE DATA and scroll down to choose OpenStreetMap.org Global Street Maps. Click Connect. Once it is loaded, zoom into the general area of your location (e.g.: southern France) using the zoom tools.
Once you have zoomed to your general area, use the Open Data Files button to add your DEM(s). For ASTER GDEM, in each unzipped folder there will be two DEM files: select the one that ends in _dem. It should show up in the correct location in Global Mapper.
Right click on the DEM (its name will start with ASTGTM) in the Control Center and select ZOOM TO from the menu. Then uncheck the box beside the DEM and the DEM should disappear.
At this point, confirm that your precise location (city, street, etc.) falls within the area your DEM just covered. If not, start again in Step 1. If you chose well the first time (hooray!), zoom into your precise location with the DEM turned off.
Once you have zoomed in to your satisfaction, turn the DEM back on. It should completely cover the screen. It will probably be a bit fuzzy because remember, it is only accurate to every block of 30m x 30m, which is almost 1 square kilometre. The contours will look fine but they will be generalized.
You are now ready to generate contours, which is in the next step. You’ve completed the hardest part and are almost done!

3: Generating contours from the DEM and exporting the result

Click on the Generate Contours icon in the menu bar, or click Analysis > Generate Contours (from Terrain Grid)... This will open the Contour Generation Options window which includes the Contour Bounds tab.
First, fill out the Contour Interval you would like. If your area is hilly (like Cahors, France), then a 1-metre interval is not recommended. However, if you have a flat area, you will probably want to get 1 metre intervals. You can do this more than once so experiment until you get your desired result. In this example, the contour interval is 5 metres. Remember: as the DEM has one elevation value for every 30m², the contours will be estimated/generalized.
Next—and this is very important—click on the Contour Bounds tab and select All Data Visible On Screen. Otherwise, contours will generate for your entire DEM which will take several moments. Click OK.
Your contours will generate (called GENERATED CONTOURS in the Control Center) and will be visible on screen. Repeat steps B and C until you are satisfied with the results, using the Control Center to toggle overlays on and off. Turn off the DEM to see what the contours look like.

4: Exporting the contours

Once you are happy with the contours, uncheck the contour layers you do not want
- Global Mapper will export all the contour lines as one file if you keep them all checked
Click File > Export > Export Vector/Lidar format...
Select your desired output format (e.g.: DWG, Shapefile, etc.). Click OK. Click OK again if you get a warning.
If exporting as DWG, make sure to click the Generate Projection (PRJ) File checkbox. This will ensure that your DWG contours will show up in the right location on the surface of the earth.
Click on the Export Bounds tab then the Draw a Box button if you want to select a particular area
Choose your file location and filename, then click Save.
Pat yourself on the back - you're done!