[mkgmap-dev] HGT - getElevation()

Tue Jan 9 14:45:14 GMT 2018

Hi Gerd,

I'm trying to find if resolution can be better. I think current 
interpolation is not good. Imagine two combination of 4-points heights 
from HGT with values like:

1 0
0 7

and

0 1
7 0

For a point in the middle of rectangle, current interpolation would give 
result 0 in first case and 4 in second, while I would expect 2 in both 
cases.

I think the code, which calculates interpolation based on 3 points, 
should be used only if the forth is void. There could be 3 combinations 
of triangles in this case. But if all 4 points are valid, there should 
be applied standard bilinear interpolation. Something like:

if (hlb == HGTReader.UNDEF) {
	return triangle(...
}
if (hrb == HGTReader.UNDEF) {
	return triangle(...
}
if (hlt == HGTReader.UNDEF) {
	return triangle(...
}
if (hrt == HGTReader.UNDEF) {
	return triangle(...
}

// 4 points valid
return bilinear(...

-- 
Best regards,
Andrzej