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Elevation disparities - Caltopo vs. Google Earth

While trying to figure out how best to import custom maps for use on my Garmin 66i, I encountered something unexpected. I used the Flood.kmz add-on (discussed by VK2GOM on his blog) to check the AZ for a summit I’ve activated before. I then overlayed in Google Earth a map I made in Caltopo that shows a successful route I took to a summit. The overlay route in Google Earth showed small route segments that cross into the AZ. However, in Caltopo, those same route segments are not in the AZ. Hovering my mouse over those route segments in Caltopo and Google Earth displays different elevations for the same locations in each tool.

I made sure my DEM shading in Caltopo used meters to match the flood.kmz units. I also changed the contour interval in Caltopo to 5 meters, and the number of contour lines counts out to 25m, consistent with the DEM shading occurs.

Has anyone else ever seen this? I was hoping to generate a KMZ map from Google Earth with Caltopo overlays that I could use in my Garmin, but now I’m not sure which tool to use even standalone given this disparity.

73 Paula k9ir

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Hi Paula,
I also encountered significant deviations between the flood tool of Google Earth and official maps. See also this post:

I’d rather trust official maps instead of Google Earth.

73 Jens HB9EKO


Wow, I missed that thread entirely when I searched before posting. Very interesting.

K6EL’s mention of using the SOTA Goat app to ID when one is in the AZ reminded me of another recent experience. I cross-checked my progress to the AZ with the elevation as shown on my Garmin 66i. The Garmin elevation suggested I already was in the AZ – after stopping to let it settle on a reading – but my Caltopo map clearly showed I still was short distance away.

I later looked up the accuracy of the Garmin – With the Auto Calibration feature turned on, the altimeter accuracy can range from +/- 50 to 125 feet (same as GPS elevation). That disparity can make all the difference between being in or out of the AZ. If you have a strong GPS signal with WAAS, you can be within 15’ - 25’ of accuracy. Closer but…Horizontal measurement is much more accurate than vertical.

Just for fun, I’m going to turn on auto-calibrate and WAAS in the Garmin and see how much difference it makes. In the meantime, it appears using the topo maps in Caltopo are more reliable for determining the AZ, so I may need to pop for a Pro membership to be able to export KMZs of my maps to use on the Garmin.

73 Paula k9ir


Google Earth flood function can give results that differ from the displayed pointer height. That is it is sometimes internally inconsistent.

I find that using a polygon larger than a couple of km across gives large errors and ignores some detail. Use with caution.

Many maps issued on line by some sources are more for entertainment than measurement. Be wary.

Government issued maps are the most reliable. Older survey details may differ from GPS surveys because of the change to WGS84 and it’s later versions.

Unfortunately the taxpayer funded maps here in VK are not available free across the country.

Goog luck.


Hi Paula,

The summit survey teams learned early on that google earth elevations are not to be trusted. This may be put down to the fact that its data came from shuttle sweeps rather than on-ground surveys. So it doesn’t account for non-natural features on summits like buildings that artificially raise the altitude of the highest point on the summit. And in other ways it is inaccurate, and was treated as an aid by sota surveyors rather than a source of truth.

Interestingly, google maps gives more accurate data, perhaps due to being based on land survey data, most likely government sourced.

When we used google earth flooding to search for prominence indicators we then had to verify the location and altitude of significant points using other mapping sources.

73 Andrew VK1DA/VK2UH
AM for VK2


Like wise as Andrew said for vk5 Google Maps satellite images where used mostly to search for likely summits and Geo Science maps were used to find closest to real altitude and prominence contours. Geo Science maps also had the name of most Summits on the map where as Google Earth missed some names. At every summit I have visited for the first time I found the trig point or survey marker then worked back down hill a way if I needed shelter from the wind with in the AZ.

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Google really does have unbridled power now eh?.

Still, when the next biblical flood happens, y’all are going to:
a) Know where to go
b) have something to do there for 40 days


In that regard, we are very lucky here in HB9.
73 Jens HB9EKO

Not across the country but in NSW this is a fantastic resource: https://maps.six.nsw.gov.au/

And here in Bavaria, Germany we have https://geoportal.bayern.de/bayernatlas/index.html

So some areas do have good maps available on-line to the general public.

On a global basis, Open Topo maps (accessible via the SOTAMaps site) is quite good.

As has been said many, many systems for altitude and prominence are based on SRTM data which has a level of inaccuracy, so use it first to see where a summit is and then switch to a good local online mapping service if you have one available.

73 Ed.

Based in SRTM unfortunately, so no good for determining AZ boundaries.

Oh, I hadn’t realised Open Topo used SRTM - that’s a shame. So it’s back to looking for local government or government agency provided online mapping where available to get the needed accuracy.

73 Ed.

"How accurate is SRTM data?

The Shuttle Radar Topography Mission ( SRTM ) Digital Terrain Elevation Data (DTED) are used with the consensus view that it has a minimum vertical accuracy of 16 m absolute error at 90% confidence (Root Mean Square Error (RMSE) of 9.73 m) world-wide.8 Feb 2017"

the link for this quote is https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5296860/

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It’s the other 10% that’s the issue :slight_smile:

We did an analysis of Nepal which is, due to SRTM, woefully incorrect. There are newer sources of data available which yield better results, even though their accuracy is quoted about the same. The reality is unless you’re using small scale LIDAR (SRTM/TandemX/etc is Synthetic Aperture Radar, not LIDAR), you’re not going to get remotely close enough, and LIDAR has its own problems Simon detailed in the other thread.

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