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How to- 20 Foot California Topo Overlay

Started by California L33, November 12, 2010, 04:06:40 PM

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California L33

I'm brand new here, and not great at searching forums, so excuse me if this tip is common knowledge, but there is a way to get 20 foot overlays for California using 'California Topo' ( which isn't listed as a transparent map. This is good for folks who have City Navigator or the old MetroGuide and want to see their named and routable (only early versions of MetroGuide) streets with the 20 foot overlays.

When I was having trouble with California Topo I did find a thread here mentioning that it wasn't one set of maps, but two. One has the road data. The other has the topographic data. Rocket scientist that I am, it took about two days for it dawn on me that if there are two maps sets shown simultaneously one of them has to be transparent. Sure enough, it's the topographic map that's transparent. (The reason you'd want to use this instead of the mc73 overlay designed as a transparent map is that the mc73 data is in 40 foot overlays, so you get twice the detail with California Topo.)

How to do it-

Install California Topo, open MapSource. Select Californa Topo on the pull down menu. Selecting only the 20 foot overlays is fairly easy. For a small area you can select maps individually. The transparent topo maps are the tall, thin, evenly spaced rectangles. It's important NOT to select any of the road maps as they aren't transparent, and if you accidentally select one you won't be able to see your other maps beneath. To do the entire state (or a large area thereof) zoom way out and lasso a big area with the map tool. To the left of the maps are the 'user data tabs.' Select 'maps.' In this window look for any maps labeled 'California Topo' that are named with place names like 'Santa Cruz,' 'Eureka,' etc. Those are the street maps. Delete those. (Not from the disk, just from that window.) Leave those that are named with an eight digit number. Those are the topo maps.

Naturally you need to also add that City Navigator, MetroGuide, or other maps you want visible under the topo data. Select those from the pull down menu and add them as usual. Send the maps to your GPSr and voila- named streets with 20 foot topo overlays. (I've only been able to test it with my Vista HCx, but it works with that.)

Special thanks to map makers. Your work is appreciated.


Quote... you get twice the detail with California Topo.

Not true.  What you have is 'twice' as many lines.

We construct the contour lines from USGS gridded elevation data.  The source of most of this data are the contour lines on the USGS 100k topo quads.  For CA, over 66% of these quads use a contour interval wider than 20 ft and over 8% use a contour interval wider than 40 ft.

In creating the gridded data, the points nearest to a source contour line are the most accurate; likewise in contouring the lines nearest to the grid points are most accurate (especially those grid points nearest to the original contour lines).   It is like 'reading' the contour lines on a printed topo - they describe the surface configuraton along the contour lines, but can only guess (within limitations) at the surface configuration between adjacent contour lines.  The gridding and contouring software programs are also guessing. 

I have seen instances where contour lines with the small upslope 'v's alternate with smooth contours.  Also one detailed contour, a wide space, three close together smooth contours, a wide space, then the next detailed contour.  In both situations  the detailed contours were at the CI (and location) of the CI's on the printed topo quad - the others were a mathametical guess.

The production method used to create the gridded data effects the quality.  USGS is continually improving the detail/quality.  In some areas of CA, the data has been improved in the past three years.

I created a mapset of my local area using a CI of half what was on the source topos.  I am aware of its usablilty/limitations.  Use whichever you feel is best for you; however, be aware that simply having a closer CI may not give you any more detail of the actual surface configuration and could be missleading.

California L33

Thanks for the clarification, though from a practical end user point of view sometimes having twice as many lines is beneficial, even if you're not technically getting more accurate data. Looking at the local biking 'heartbreak hill' (quite short and steep) on Garmin's US Topo and you can barely tell there's a hill there. Look at it with 20 foot topo lines and you can tell it's a leg breaker.


Agreed.  I liked my 20f CI when hiking on the local mountain trails; crossing contour lines more often seamed to be making faster uphill progress even if there were twice a many lines to cross.

Garmin's US topo is a 1:100,000 scale product.  The mapset with 20ft contours is likely built from 1:24,000 scale data.  As in the paper products, a 24k will show more detail than a 100k.  On the paper maps, USGS uses a CI appropriate to the map scale and steepness of the topography on a per quadrangle basis.  They also may use carrying contours in very steep areas to keep from haveing a blob of tan ink and/or supplemental contours to add detail to the gentler sloping areas within a quadrangle. 

I heard some European bikers are contouring the 90m resolution SRTM data at a 5m CI.  Given the technical specs of that data, a 30m CI would be more appropriate. 

There have been comments that the contours on that mapset are so overpowering that they can not easily see the other details of the mapset, and therefore it is of little value to them.  If the 20ft contours are more sutable for what you need, by all means use that mapset.

Indrid Cold

There are some problems with the contours on that mapset that may or may not effect your use such as no contours at or below 0' (see Salton Sea or Death Valley) and I believe there are some quads on the Nevada border missing contour data near Lake Tahoe。