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Refresh of Elevation data for Shetland needed

+6 votes
149 views
Looking at the top elevation report there are currently 164 caches >450m in UK/Northern Scotland/Shetland. The problem is that Shetland is very flat, the highest point in Shetland is only 450m yet the database is showing numerous caches at 4000+m this is wildly out.

I would recommend a refresh of the cache data for Shetland. Note there were about 200 caches published on a single day there so it could be that there was a glitch in the database around that time and all the caches newly published were affected.
asked Jul 28, 2017 in Bug reports by ShammyLevva (Expert) (8,230 points)
This was raised again on the Geocaching Scotland Facebook group. I did some further research and noted a particular issue.

The STRM3 data that is being used does not exist above N60 20.000 unfortunately Shetland straddles N60 20.000 and it appears that it's the caches north of this point that are falling foul of the problem.

My suspicion is that it is the border cases that are inaccurate. Proof of this might be obtained by looking at Norway/Sweden/Finland which I'd imagine the site owner might have a better familiarity with :)

In any case the data north of N60 20.000 for Shetland is wildly inaccurate by around 4000 yes four THOUSAND meters. So whatever alternate source is used for non STRM3 areas there is a huge problem.

2 Answers

+2 votes
This is actually something we thought we fixed quite a while back. To look at it today, I took a random gccode from UK which had a high elevation, GC2CRWZ.

Our GetElevation routine has three different services it relies on. SRTM3 is the first, it returns false for GC2CRWZ. It then fallbacks to Google services, where we have a rate limit.

The third variant is AsterGDAM data, which is very jumpy. When it works, it's great, but when it's off, it's usually off with a lot.

A while back, we had another service between our local SRTM3 data and Google. That service isn't open anymore and therefore not used. We also had AsterGDAM before Google, which caused many issues.

When we moved AsterGDAM to the last in the routine we also ran an update on all geocaches we though affected. Either we missed a few, or we ran into rate limits with Google services so that our services fall back to AsterGDAM again.

We are currently running an update on all geocaches with latitude > 60 and elevation > 500. From what I can see, all in UK is already fixed.
answered Nov 10, 2017 by magma1447 (Admin) (208,230 points)
Hmm I just ran https://project-gc.com/Statistics/TopElevation?country=United+Kingdom&region=Northern+Scotland&submit=Filter

and it is still showing 78 caches >1325m (which is the height of Ben Nevis the highest mountain in the UK) all of those >1316m are wrong. There are some archived ones showing 1324m which is fine as 8m is nothing to worry about. https://coord.info/GC161P6 is the highest active cache in the UK.
The cache GC2CRWZ is archived but is right on the shoreline of the island the highest nearby point within 100m is 119m. So the whole area is relatively flat.

I took a screenshot of the OS map of that cache area showing the contour lines (I stuck a red dot to show where GC2CRWZ is) and saved the screenshot to my public dropbox folder at https://www.dropbox.com/s/5dn8krp27m22271/ShetlandCacheIssues.PNG?dl=0
The data you see isn't the latest in our database, it will take a few hours before it's available in Top Elevation. However, we did not get all data updated, we hit our rate limits rather fast with the Google API. It allows 2500 requests per day, where one request can include quite a few coordinates. However, our code does not bulk it, because it's not compatible with other services.

I just found that OSM has made a new service, September this year. I will implement support for that today.
+1 vote
 
Best answer
We made a few changes after this post, and it caused a lot of issues, elsewhere. That led us to rewrite the flow we had. We are still in the process of updating elevations. About 5% done in ~24 hours.

Covered in today's newsletter: https://project-gc.com/Home/News?goto=100

FAQ entry describing the new flow: https://project-gc.com/Home/FAQ#4179902386
answered Nov 21, 2017 by magma1447 (Admin) (208,230 points)
selected Nov 21, 2017 by ShammyLevva (Expert)
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