I'm trying to estimate the elevation at given point. Since the estimation has to happen in real-time, I'm willing to compromise in the resolution by having a min and max elevation returned instead. I thought that I would start by creating a dataset using Google Earth Engine by dividing the earth roughly into 10 by 5 degree chunks.
Then I iterated over each chunk and added min/max elevation to each chunk using SRTM Digital Elevation Data Version 4. What I noticed is that if the chunk contains large areas of water the min/max is missing from the result. For example the following code calculates the elevation around Norway
var elevation = ee.Image('CGIAR/SRTM90_V4').select('elevation');
var createRectangle = function(lon, lat, dLon, dLat) {
var lon1 = ee.Number(lon).subtract(dLon / 2);
var lat1 = ee.Number(lat).subtract(dLat / 2);
var lon2 = ee.Number(lon).add(dLon / 2);
var lat2 = ee.Number(lat).add(dLat / 2);
return ee.Geometry.Rectangle([lon1, lat1, lon2, lat2]);
};
var rectangles = [];
var dLon = 10;
var dLat = 5;
for (var lon = 2; lon < 20; lon+=dLon) {
for (var lat = 59; lat < 65; lat+=dLat) {
rectangles.push(createRectangle(lon, lat, dLon, dLat));
}
}
var elevationStats = ee.FeatureCollection(rectangles.map(function(rectangle) {
var statistics = elevation.reduceRegion({
reducer: ee.Reducer.minMax(),
geometry: rectangle,
scale: 30000
});
var minElevation = ee.Number(statistics.get('elevation_min'));
var maxElevation = ee.Number(statistics.get('elevation_max'));
return ee.Feature(rectangle, { min: minElevation, max: maxElevation });
}))
Map.addLayer(elevationStats, {color: 'red'}, 'Elevation');
The 2 top most rectangles west coast of Norway have no elevation data where the bottom ones do. I figure that the SRTM90 does not have elevation for "ocean" but all of the chunks contain some land.
Am I using the Reducer wrong?
Edit 1: I just noticed that the SRTM Digital Elevation Data Version 4 does not contain data above latitude 15... "high-quality elevation data at near global scope" indeed
