Acquiring & Converting DEM Data in VMG Gasmod
One of the powerful features in VMG Gasmod that expedites model creation is the ability to import topographic data and then automatically subdivide a pipe network to honor that topography. There are US and Canadian websites (usually governmental) that provide free Digital Elevation Model (DEM) data and VMG Gasmod has been developed to accept this type of file format.
There are multiple DEM file formats so conversion is sometimes required. This article provides the user the steps to acquire DEM data and, ultimately, import it into VMG Gasmod.
If topographic data is available in your area but not in DEM format, it might still be possible to convert it into DEM format using the procedure in Section 3 below.
Acquisition of Canadian DEM Data
- For Canadian DEM information the data from the following website has proven useful.
- Go to the above website and Find a Location or using the map, select a Clipping Region of interest.
- Click Select Data and choose Elevation Model under Elevation Data; the region of interest will be highlighted.
- Under Select Data options choose DEM (your only option is a GeoTIFF).
- Click Submit Request, enter your email address, click Submit and you will be emailed a link to download the data.
- Click the link in the email you receive and select a location to save the *.zip. Browse to that location and extract all of the files. In the *.tif folder the *.tif file exists.
Acquisition of US DEM Data
- For the US DEM information, the 1-deg DEM Terrain Data at the following site has proven useful: http://www.webgis.com/index.html
- Go to the above website and select Digital Terrain data-1-deg DEM.
- Select the State you want DEM info for through the map or list view and then choose the county.
- Click on the tile(s) (*.zip file) you wish to download. Once downloaded, extract the *.DEM file to a preferred location.
Conversion to VMG Gasmod Compatible File
- Open OSGeo4W QGIS Browser - 32 bit version (available at no cost from https://trac.osgeo.org/osgeo4w) and browse in the left window to the location of your extracted file.
- Double click the file and it should be loaded as a layer into QGIS.
- Go to the Raster - Conversion - Translate menu.
- The input layer should be populated with the existing layer file. On the Output File line, click Select and browse to where you want to save the converted *.DEM, give it a desired name and make sure to set Save As type to USGS Optional ASCII DEM.
- Enure the Target SRS line is set to ESPG: 4269. Click OK and the conversion will take place.
- Ensure that you shut down QGIS before trying to import the converted *.DEM into VMG Gasmod otherwise it may fail.
Import into VMG Gasmod
- Open the VMG Gasmod project and select File - Import Data - DEM.
- Browse to the converted *.DEM file and select it. VMG Gasmod should import the *.DEM info and create a shape file.
- The above steps can be repeated to cover areas of interest that span multiple tiles.
Saving DEM Info for Network Extents
- To reduce the size of the project and *.DEM file, the ability to create a *.DEM file for the region locally surrounding the pipeline network is available. Once the desired *.DEM info has been imported into VMG Gasmod as per the steps above and the gathering system has been drawn, go to File - Export Data - Network Extents DEM. This will export a *.DEM with boundaries just outside the limits of the pipeline network. It will be located and named in the project folder as C:\VMG\”project name”\”project name”.DEM. This file can be imported into the existing project or others.
Please contact your local VMG office for more information.
Gordon Slemko, P.Eng., VMG Calgary