Do Van Der Grinten I and Van Der Grinten II uh... not have the middle Indicatrix?

[Piotr]

Geocart does implement some inverse formulæ, generally for the most commonly used projections. In a few cases, the Tissot indicatrix and other analytics also implemented explicitly (such as for the ellipsoidal transverse Mercator). Everything else is approximated using numerical techniques.

Is that why many blends fail?

Most blends that fail do so either because (a) the topology has overlaps, or (b) Geocart wasn’t able to synthesize a coherent boundary. The boundary synthesis is complicated and the code is buggy.

If the topology does not overlap itself and if Geocart can synthesize a usable boundary, then the blended projection will normally succeed.

— daan

So blending a blended projection with another fails because boundary synthesis cannot handle synthetic boundaries?

[daan]

If the topology does not overlap itself and if Geocart can synthesize a usable boundary, then the blended projection will normally succeed.

So blending a blended projection with another fails because boundary synthesis cannot handle synthetic boundaries?

Blending two projections, at least one of which is already blended, fails because Geocart does not recognize the identity of the blended projection as expressed by its name. That’s probably not hard to fix, but until blending is more robust, fixing it wouldn’t result in a lot of successes. Synthesis on top of synthesis would compound the brittleness.

— daan

[Piotr]

If the topology does not overlap itself and if Geocart can synthesize a usable boundary, then the blended projection will normally succeed.

So blending a blended projection with another fails because boundary synthesis cannot handle synthetic boundaries?

Blending two projections, at least one of which is already blended, fails because Geocart does not recognize the identity of the blended projection as expressed by its name. That’s probably not hard to fix, but until blending is more robust, fixing it wouldn’t result in a lot of successes. Synthesis on top of synthesis would compound the brittleness.

— daan

I see, it has to verify the original projections are authentic, because you know, blending hacked projections could result in security exploits...