The Open Elevation Service is extremely simple to use. You just submit a URL containing the location:
http://open.mapquestapi.com/elevation/v1/getElevationProfile?callback=foo&shapeFormat=raw&latLngCollection=39.740112,-104.984856
and you get back a JSON (or XML) response with the elevation:
foo({
"shapePoints":[39.740112,-104.984856],
"elevationProfile":[{"distance":0,"height":1616}],
"info":{"copyright":{"text":"© 2011 MapQuest, Inc.","imageUrl":"http://tile21.mqcdn.com/res/mqlogo.gif","imageAltText":"© 2011 MapQuest, Inc."},
"statuscode":0,
"messages":[]}
});
JEQL doesn't (yet) do JSON parsing, but it's easy to pick out the height attribute with a simple RegEx:
height = RegEx.extract(elevJson, \'"height":(-?\d+)\}');
Querying over a grid of locations provides a dataset which is a gridded Digital Elevation Model (DEM). JEQL provides a function to generate a table which is a grid of integers, so it's easy to produce a grid of lat/long locations:
loc = select baseLat + i * cellSize lat, baseLon + j * cellSize lon
from Generate.grid(1, gridSize, 1, gridSize);
Once the elevations for all locations are retrieved, the locations are used to compute square cells (polygons) which cover the area, and the cells are symbolized with a fill value from a suitable color map based on elevation.
Here's the final result. It's slightly squashed in aspect ratio, since the data grid is in the lat/long coordinate system. The obvious data dropouts seem to be hard errors in the MapQuest service, which is slightly surprising. But's it's recognizably Mt. St. Helens - you can even see the lava dome in the crater!
Not surprisingly, this isn't quick to generate - the service runs at about 10 queries per second. (Actually, that seems pretty reasonable for a free web service!). The service does support querying multiple points per request - I might try to see if this decreases the latency (although this might be a bit complex to express in JEQL).
Here's the whole script:
//--------------------------------------
//
// Plots a DEM generated using the MapQuest Elevation service
//
//--------------------------------------
baseLat = 46.16 ; baseLon = -122.25; // Mt St Helens
areaSize = 0.10;
gridSize = 100;
cellSize = areaSize / gridSize;
//---- Generate location grid
loc = select baseLat + i * cellSize lat,
baseLon + j * cellSize lon
from Generate.grid(1, gridSize, 1, gridSize);
//---- Query elevation service for each grid point
telev = select lat, lon, elevJson, elev
with {
url = $"http://open.mapquestapi.com/elevation/v1/getElevationProfile?callback=foo&shapeFormat=raw&latLngCollection=${lat},${lon}";
elevJson = Net.readURLnoEOL(url);
height = RegEx.extract(elevJson, \'"height":(-?\d+)\}');
elev = Val.toInt(height);
}
from loc;
Mem telev;
//---- Create raster cell boxes, symbolize with color map based on elevation, and plot
minElev = 800.0; maxElev = 2500.0;
tplot = select Geom.createBoxExtent(lon, lat, cellSize, cellSize) cell,
Color.interpolate("00a000", "ffb000", "aaaaaa", "ffffff", index) style_fillColor
with {
index = (elev - minElev) / (maxElev - minElev);
}
from telev;
textent = select geomExtent(cell) from tplot;
Plot extent: val(textent)
data: tplot
file: "dem.png";
1 comment:
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