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[mkgmap-dev] HGT - getElevation()

From Frank Stinner frank.stinner at leipzig.de on Wed Jan 10 09:53:44 GMT 2018

Hi Gerd and Andrzej,

> your choice of triangle is arbitrary.

of course, but the version with 4 triangles isn't.

> bilinear interpolation seems better

Yes. It's a little bit more to calculate, but better. For our special case 
it is: 

h = h11 * (1 + y * x - x - y) + h21 * (x - y * x) + h12 * (y - y * x) + 
h22 * y * x

(h11 is left-bottom in coordinate origin, the horizontal and vertical 
distance is 1)

If your java-class have to much overhead, you can use this formula.

By the way, i would be very defensive with interpolation in the case of 
missing values. The hgt values are only interpolated values from original 
measurments. I would not interpolate values in the near of missing values. 
That give us a nice but unreliably picture.

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