

Google Maps Elevation Web Service - drp
http://code.google.com/apis/maps/documentation/elevation/

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cullenking
Cool service, albeit it's for smaller uses. The limit is 25,000 requests a
day. This sounds like alot, until you realize each point is a request. So, you
want elevations for a 100 mile route drawn on a map? That's about 2-3000
points, so you only get 10-20 of such routes before hitting your daily limit.

Other issue is the accuracy: I am not sure the base DEM resolution they are
using, but my guess is it's the standard 10meter spacing between points inside
US and territories, and 30meters outside. 10meter grid spacing isn't bad at
all, it's actually pretty good, but their algorithm for interpolating is non-
existent. For any point not directly on a grid point, they just average the
four surrounding elevations and return that. I have played with several
interpolation schemes, and I have found either a bilinear or a cubic
interpolation for interior points to be vastly superior to a nearest neighbor
or averaging scheme. Differences can be easily up to 10 meters for a 10meter
grid spacing.

I had been hoping they'd release this, but I'll stick to my home-brewed
service with the 500gig NED at 10 meter resolution (1/3 arcsecond). No API
limits :)

~~~
dougmccune
[Edit: as pointed out below it seems like it really is 25,000 unique
locations, my mistake.]

 _so you only get 10-20 of such routes before hitting your daily limit._

Not true, from the usage docs: _Up to 1,024 locations may be queried within a
single request._

So that's really a theoretical max of 25,600,000 locations per day if you
maxed out every request.

~~~
borism
_Not true, from the usage docs: Up to 1,024 locations may be queried within a
single request._

"Note that on any given query, you may request elevation data for multiple
locations, which count as separate queries."

Approximate! No need to do a lot of requests for flat land!

~~~
cullenking
Approximations only get you so far. Try telling picky cyclists, who compare
your calculated elevation gain/loss to ten other sources out there, that you
just want to approximate :)

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samueladam
The NASA Shuttle Radar Topographic Mission (SRTM) has provided digital
elevation data (DEMs) for over 80% of the globe. This data is currently
distributed free of charge by USGS and is available for download from the
National Map Seamless Data Distribution System, or the USGS ftp site. The SRTM
data is available as 3 arc second (approx. 90m resolution) DEMs.

<http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp>

~~~
borism
I guess it's the same enhanced SRTM data that Google uses?

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kanwisher
Wonder if this would be accurate enough for a GPS -> bike route translator.
Like say you want to see the altitude throughout your bike right from the gps
you recorded.

~~~
akgerber
The RideWithGPS people appear to do this: <http://ridewithgps.com/trips/22776>

Though if you're somewhere very hilly ( _ahem_ Pittsburgh) it appears that
slight issues with the GPS track will throw in a lot of extraneous elevation
data.

~~~
cullenking
We are working on some cool ways to fix GPS tracks that have drop-outs in
signal/accuracy. I am going to use some signal processing tricks to smoothout
the points along the route. I am using a Gaussian filter on the elevation
data, and I think this will also work good on the actual lat/lng points, which
should also help with total gain/loss accuracy.

