Topo Map of California for Garmin GPS

A comment about map accuracy

We should expect small offsets between your GPS fix and the GPS maps. If for example you are standing on a trail intersection under clear sky with an excellent GPS fix, and the GPS map says the trail intersection should be (say) 10-20 yards away -- sorry but that's all we can expect. If you are under heavy tree canopy you can expect an even larger offset, partly because your GPS is less accurate but also because the trail location on the USGS map itself that this derives from is less accurate. Now if the offset is more than 50 yards and you have an excellent GPS satellite fix under clear sky, I want to know about that. Tell me the UTM and datum, what feature is offset, and the direction and distance of the offset. Pan over using your GPS pointer to measure the distance of the offset.

More details for those who are interested....

• You no longer need to have Garmin Topo USA. All of the topo maps are custom.
• Two custom topo maps superimpose: contour maps and roads/trails/powerlines/water maps.
• Missing on these GPS maps compared to the USGS topo maps are: most labels, vegetation type, boundaries, and man-made structures.

Contour maps

The contour maps are made from USGS DEM data files (digital elevation model). These are elevation values on a 10 meter grid covering the USA, data derived by reading elevation data off of USGS topo maps. For most of California, the DEM data were made from the 1:24K USGS topo maps that we all use. For small sections of the state, the USGS doesn't yet have DEM data files from 1:24K maps. For those sections, the DEM data are based on 1:100K USGS topo maps, and we lose some resolution of the terrain features. Most of the central valley is based on 1:100K DEM data, but that is flat anyway so it doesn't matter. There are also other patches here and there based on 1:100K DEM data. If you want to know where, send me an email and I'll forward you a map showing what parts of the state are based on 1:24K versus 1:100K DEM data.

The USGS 1:24K topo maps for California have a mismash of different contour intervals... 20 foot, 40 foot, etc. There are a small number with a 10 foot contour interval as well as a number of them with a contour interval in meters. It was too unwieldy to try to make the custom topo maps match the contour interval of the USGS 1:24K topo maps. To simplify this I picked the same contour interval for the whole state, so the GPS contour maps have a 20 foot contour interval everywhere. This means if you are in an area where the USGS maps have a 20 foot contour interval, then the contour lines on your GPS will appear nearly identical to those on the USGS map (a really nice feature for quickly orienting yourself). If you are in an area where the USGS maps have a 40 foot contour interval, then the contour lines on your GPS will appear similar to those on the USGS map but there will be twice as many contour lines on your GPS.... those nearly identical to the ones on the USGS map PLUS interpolated lines in between. Don't get excited because there is no additional resolution of terrain features even though there are twice as many contour lines.

If you move your GPS pointer to a contour line it will display the contour level.

If you zoom out on your GPS you will only see the 100 foot major contour lines, and if you zoom out further the contour lines disappear. If you zoom out farther still only Garmin's built in Base Map appears along with the outlines of the custom maps. You can adjust at what zoom levels these transitions occur on the menu Setup Map, Detail (select either most, more, normal, less, or least)

FYI: 95% of the work to make the custom topo maps was making the contour maps. After downloading the 23 GB of DEM data from the USGS website in over 300 individual files, it took 2-3 PCs crunching numbers nearly 24/7 for several weeks to generate the contour maps.

Roads/trails/powerlines/water maps

The roads/trails/powerlines/water maps are made from USGS DLG SDTS data files, based on 1:100K USGS maps. The only exception is a subset of the SF Bay Area which uses 1:24K USGS DLG SDTS data. The USGS has provided only a minority of the state in 1:24K USGS DLG SDTS data format, and it's an extremely complex patchwork. Until USGS provides a lot more of the state in 1:24K DLG SDTS format, it's too difficult to integrate the patches of 1:24K and 1:100K data together.

I have found that it's more important that the contours rather than the roads/trails/powerlines/water features are based on 1:24K USGS map data. The difference between contour lines on 1:100K and 1:24K USGS topo maps is very significant, while roads/trails/powerlines/water features on 1:100K and 1:24K USGS topo maps are more similar. For an example comparing 1:24K and 1:100K USGS DLG SDTS road & water data, check here http://www.robomatt.com/maps/map_accuracy.htm

At certain GPS zoom levels the Base Map appears while at other times at the same zoom level the custom maps appear. I don't have any control over this, it may be a glitch in the Garmin software. It only appears when zoomed back pretty far so I don't think it will impact anybody.



Author: Laura Sanborn
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.
No warranty is expressed or implied. Provided on an "as is" basis. Use at your own risk.