How To Create Garmin Topo Maps - Part 2 - Elevation Data
Author: Dan Blomberg | Last updated April 14th, 2013 at 12:30pm
This part details how to download high-quality DEM (digital elevation mapping) data from the USGS National Map system (the old Seamless Server has been closed). This is a continuation of the "How To" discussing how to create detailed Garmin topographic maps. In part 1 we discussed how to setup your computer to get ready to download and process the data.
Some software is required and some is reccomended for downloading and processing elevation data:
- High speed internet (don't even try it with dialup)
- Modern browser (IE 7+, Firefox, Safari 3+, Opera 8+, Chrome, etc)
Using The National Map Viewer to Download the Data
- Open the National Map Viewer
- First, on the left side open the Reference Polygons list
- Under Governmental Unit Boundaries enable County or Equivalent
Click the download data button
Change the reference area in Download Options to States
- Click on the state where you want to download the data from
- The state will appear on the left side
- On the left side click "Download"
- Check the box for elevation and click next
- Select the 1/3 arcsec data in either ArcGrid (for GlobalMapper) or GridFloat (also works with GlobalMapper)
- Click next
- Now on the left side click checkout
- Enter your email address and click "Place Order"
- Wait for links to your download files to arrive.
- Download each of the files; there will likely be multiple files for the state (Arizona has 41)
Downloading The Data From Seamless (Old)
Note: The seamless server is no longer operational. This section is left for archival purposes until it is determined how to retrieve GeoTiff files for dem2topo.
- Open the excel spreadsheet for your state that was downloaded in part 1 of this tutorial (Didn't get it? Get it here)
- Go to the first USGS DEM Link (cell J2), click it, then copy it (CTRL-C)
Go to your internet browser (Internet Explorer, Firefox, Safari, etc) and paste (CTRL-V) the copied URL into the address bar. A page similar to this should appear:
A normal 100k quad will have 4 quads in ArcGrid format, if you don't have 4 reload/refresh the page. If 4 still don't appear its possible you don't have 4 segments in this quad. This happens near coasts and borders of the United States. GeoTiff will have as many files as needed when the file limit is 250mb.
- Decide whether you will be using Global Mapper or Dem2Topo. If you are using Global Mapper use the first set of download links that are in ArcGrid format; if you are using Dem2Topo use the second set in GeoTiff format.
Click the first download link. A popup will appear. You have to wait for it to prepare your data. In a minute or two you will be presented with the "Save as..." dialog box. Select run/open.
Note: if you didn't get a pop-up your popup blocker is enabled. Add usgs.gov to the list of allowed sites.
- While the first one is downloading you can click to download the other segments. Sometimes only two can be actively downloaded at a time.
- Once the first zip file is open; open your computer and browse to your state folder (in my case mississippi_topo) then to the dem folder. Drag all the contents of the zip file to that folder.
- Once you have all 4 segments downloaded go to the excel spreadsheet and put an "X" in the DEM box for that quad. This is how we will keep track of the progress we have made and know where to continue from if we take a break.
- Repeat these steps for every quad for your state. Once you have all the data you can start to process it.
Processing The Data
This section includes two sets of directions. One for using the Global Mapper software and another for using the free Dem2Topo software.
Global Mapper Directions
Make sure you used the USGS DEM LINK (ArcGrid) for Global Mapper.
- Start Global Mapper
- Go to File> Create new map catalogue
- Go to your state's folder (mississippi_topo) then the dem folder. Save the map catalouge as elevation.gmc
- In the next box that appears select "Add Directory"
- Select the dem directory in your state's folder.
- For file masks enter: w001001.adf
Press "Ok" and you should get a window with a bunch of w001001.adf files similar to this:
- Press "Ok" and all the DEM files will load. They will not really be visible because of the setting to not show unless the segment takes up 10% of the screen.
- Now we are going to process the contours and export them in one operation. This operation could take as long as 36 hours depending on your computer and state (the Grand Canyon took 4+ hours on its own). Click File> generate contours.
For contour internal enter 40 ft.
Note: For Mississippi I am using 10-20ft contour intervals because that is what the 24k maps have and the state is relatively flat.
- For resolution under x-axis enter: 0.000104605841386575
For resolution under y-axis enter: 0.000081169112836251
Entering these resolution values appears to give a good smoothness versus size ratio.
- Make sure interpolate and smooth contours are checked. Uncheck Append Unit Labels to elevations.
- Check export contours directly to package files.
Now go to the gridding tab on the top. If you have a very mountanous state I would reccomend sticking to the 100k quad size so that the segments can be used in older GPS receivers. If you have a very flat state you can go larger. The 100k segment size is: 1 arc degree in width and 0.5 arc degree in height. For flat states you can use 1 arc degree by 1 arc degree.
- Click "Ok" and call the files contours
Let it process.
- If your computer crashes or you stop processing open the catalogue we created and then check the box under gridding that says "skip existing files". This will allow Global Mapper to start off where it ended.
Make sure you have the IDL Runtime installed so you can run Dem2Topo. Make sure you used the USGS DEM LINK (GeoTiff) for Dem2Topo.
- Open Dem2Topo (click dem2topo.sav)
- Select contour interval feet
- Major interval set to 400
- Intermediate interval set to 200
- Minor interval set to 20
- Disable plot and bitmap (this will save a lot fo computing time for these large segments)
- Click "select DEM file(s)
- Find the tif image for the first segment and load it (this could take a very long time, be patient)
- Click "create mp file"
- Wait for it to process the data.
- Repeat these steps for each segment.
Now that our contour data is all downloaded and processed we will start downloading the water data. This is another time consuming process but for slightly different reasons. Expect that in a week or so.