Last week we had a look at drawing contour lines using the Google Maps Elevation API. Because of restrictions on what you are allowed to do with Google’s elevation data, we chose not to create KML files from it. We have since talked to another elevation API provider, Mapzen and they assure us that they use open data from USGS and NOAA and we may do whatever we like with it.
Google’s elevation data, as used in Google Earth is higher resolution in many locations than the Mapzen data because Google also has access to the USGS and NOAA data, but have supplemented it with other sources that are not as open.
Here is an example of what is possible with the new version of our tool:
To try it out with default settings, just click the ‘Draw Contour’ button and see what happens. The default settings should take about 30 seconds to complete. When it is done, it shows the result in the Google Map below as well as downloading a KML file, so you can view the contours in Google Earth.
To use it with your own settings, draw a polygon in Google Earth of the approximate area you are interested in, save it as a KML then select it below. If you do not select a KML file then it will default to using an area in British Columbia, Canada that we selected for its very mountainous terrain.
You can choose how many rows and columns of altitude data to obtain. The Mapzen elevation API limits the number of queries you can make in a given time to prevent individual users from degrading the overall system performance, so very high resolutions take a long time. Remember that doubling the rows and columns will take four times as long to complete.
Selecting ‘Show Altitude Grid’ only works in conjunction with the ‘Single contour’ mode. It displays the grid of the altitudes that were obtained from the Mapzen API and shows them in red if they were above the selected altitude, or green if they were below the selected altitude. Zoom in to see how they relate to the contour line.
[ Update: Also see this post regarding improvements we made to the algorithm. ]
Show Altitude Grid
Altitude: m above sea level
Contour every: m
If you don’t like the styling of the contour lines in the KML, you can change them in Google Earth from the properties of the containing folder rather than having to modify each individual contour.
Let us know in the comments if you find any bugs or have suggestions for improvements. We would also love to know if anyone finds this tool useful for anything.