Google Maps API Tutorial

Altitude

Here's a simple example

One place where you can get altitude information is the US Geological Survey Elevation Query. Information about their service can be found here:
gisdata.usgs.gov/XMLWebServices/TNM_Elevation_Service.php

A request looks like this:
http://gisdata.usgs.gov/xmlwebservices2/elevation_service.asmx/getElevation?X_Value=-118.4&Y_Value=36.7&Elevation_Units=METERS&Source_Layer=-1&Elevation_Only=true.

The parameters are:

X_Value	Longitude
Y_Value	Latitude
Elevation_Units	METERS or FEET.
Source_layer	You can use this to request data from a specific survey, or use -1 to request the best available data at the location.
Elevation_Only	true or false. If you set it false you get information about the data source as well as the elevation data.

The service provides a SOAP interface that returns XML, so you can't access it directly from Javascript for security reasons. What you have to do is to write a little server script that runs in your own domain. Your Javascript can send a request to your own server, which then sends the request to USGS and returns the reply back to your Javascript client.

I'm not a PHP expert, but I managed to throw together something that works for this purpose. A general purpose relay script would need to be more complicated than this, to avoid the possibility of cross-domain attacks, but I think I can trust the USGS. My PHP code looks like this and returns the data like this: altitude.php?lat=53&lng=-2.

There are two asynchronous steps in this chain, so don't expect to be able to write a function that returns the altitude of a point. Send the request with GDownloadUrl() and then process the reply in the callback funtion. If you're sending several requests (e.g. to obtain the profile of a cycle route) you can wrap your call in a function and use it to hold Function Closure so that you can match the replies back to the requests.

The service will return up to 10 decimal places of information, but I suspect that the surveys aren't accurate to 0.1 nanometres. In my code I just use the integer value.

The service returns the value 0 for open sea. You could therefore use it to distinguish land from sea, but there are a few other points on land where the elevation is also zero, e.g. Elburg in the Netherlands.

The documentation states that the service can return the value -1.79769313486231E+308 if there is no valid data for the requested location, so you might want to filter out such values if you're drawing an elevation diagram.

In this example I use GDirections to find a route, use the EPoly extension to obtain points at equal distances along that route, use USGS to find the altitude at those points, and use the Google Chart API to plot a chart of those altitudes.

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