Unclear behaviour of param +nadgrid= vs. +nadgrids=

[lczub]

I find following not clear behaviour of cs2cs during a typing error.

I was surprised, that the declaration with the wrong parameter '+nadgrid=' has another effect as the declaration of the correct parameter '+nadgrids=' with a non existing NTv2 file (see test b+c vs d).

My expectation was, that test b+c+d should use the same default datum shift parameter, but this is not the case.

A description for '+nadgrid=' could not be found in  ProjAPI. What is the impact of this parameter?

Test A: declaring existing ntv2 file with correct param +nadgrids

Expectation: 399340.601863 5928794.177992 -> ok

C:\>cs2cs +init=epsg:31466 +nadgrids=BETA2007.gsb +to +init=epsg:25
832 +towgs84=0.0,0.0,0.0
2598417.333192 5930677.980308
399340.60       5928794.18 0.00  
^C

Test B: declaring existing ntv2 file with typing error param +nadgrid

Expectation: great delta to Test A, cause wrong param nadgrid should be interpreted as missing datum shift -> why is the delta so small?

C:\>cs2cs +init=epsg:31466 +nadgrid=BETA2007.gsb +to +init=epsg:258
32 +towgs84=0.0,0.0,0.0
2598417.333192 5930677.980308
399339.28       5928789.39 -5.76 
^C

Test C: declaring non existing ntv2 file with typing error param +nadgrid

Expectation: great delta to test A, cause wrong param nadgrid should be interpreted as missing datum shift -> why is the delta so small?

C:\>cs2cs +init=epsg:31466 +nadgrid=nofile.gsb +to +init=epsg:25832
 +towgs84=0.0,0.0,0.0
2598417.333192 5930677.980308
399339.28       5928789.39 -5.76 
^C

Test D: declaring non existing ntv2 file with correct param +nadgrids

Expectation: great delta to test A, cause missing ntv2 file should be interpreted as missing datum shift -> ok

C:\>cs2cs +init=epsg:31466 +nadgrids=nofile.gsb +to +init=epsg:2583
2 +towgs84=0.0,0.0,0.0
2598417.333192 5930677.980308
399399.12       5928964.19 0.00 

[warmerdam]

Hi,

Test (B) and (C) with +nadgrid is operating using the +towgs84 parameter implicit in 31466. Misspelled parameters are essentially ignored.

Test (D) applies no grid shift but because it attempted to find nofile.gsb, the +towgs84 implicit in 31466 parameter is ignored.

I'm somewhat surprised that case (D) did not report an error, but otherwise things are operating as expected.

The +nadgrids parameter is described in General Parameters topic.

I'm not clear on any action item for this ticket.

Sorry for the slow response.