gdal_contour skips first contour when on edge

[jidanni]

Here the 0.5 is missing. Only 1 and 1.5 were output. Gdal 3.9.

gdal_create -q -outsize 2 2 a.xyz
perl -awnle '$F[-1]=$F[0]; print "@F"' a.xyz > d.xyz
cat d.xyz
0.5 -0.5 0.5
1.5 -0.5 1.5
0.5 -1.5 0.5
1.5 -1.5 1.5
gdal_contour -q -i 0.5 -a Z d.xyz d.csv
cat d.csv
ID,Z
0,1
1,1.5

And it makes just as little sense as if 1.5 was missing, and only 0.5 and 1 were output.

Nor does the man page document that the first contour is skipped if there are no data points below it.

Which is good, as documenting that would be just as bad as saying that the last contour is skipped if there are no data points above it.

If contours on edges are to be skipped, then at least skip consistently.

But that would leave us with only one contour, at 1.

Solution: fix the bug that arbitrarily skips the first contour, even though there are two or more data points right on it, that should become part of a new LINESTRING contour.

Else it is like buses that run every half hour from 8:30 AM to 5 PM (but oops, not the first bus.)

[jidanni]

I also worry that using -fl <first contour> explicitly might not work either, so do please check it. Thanks.