NTv1 grid shift: fix file offset for reading of shift values in ntv1_can.dat #1144
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…can.dat When investigating the format of NTv1 and comparing PROJ code with the actual header of ntv1_can.dat, I discovered that the longitude & latitude shift values started at offset 192, whereas PROJ assumed that the header was 176 bytes only. This caused PROJ to use the wrong offsets values (shift of one grid sample by longitude). So the effect was moderately visible, especially on the latitude, but when comparing with NTv2, one can see that the longitude value after the fix seems to closer to NTv2. old: echo "60.5 -100.5 0" | PROJ_LIB=/usr/share/proj src/cct -d 8 +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=hgridshift +grids=ntv1_can.dat +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1 60.50022624 -100.50040292 0.00000000 inf new: echo "60.5 -100.5 0" | PROJ_LIB=/usr/share/proj src/cct -d 8 +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=hgridshift +grids=ntv1_can.dat +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1 60.50022403 -100.50041841 0.00000000 inf echo "60.5 -100.5 0" | PROJ_LIB=/usr/share/proj src/cct -d 8 +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=hgridshift +grids=$HOME/proj/proj-datumgrid/north-america/ntv2_0.gsb +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1 60.50022348 -100.50041978 0.00000000 inf old: $ echo "80.1 -70.9 0" | PROJ_LIB=/usr/share/proj src/cct -d 8 +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=hgridshift +grids=ntv1_can.dat +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1 80.10096789 -70.89746834 0.00000000 inf new: $ echo "80.1 -70.9 0" | PROJ_LIB=/usr/share/proj src/cct -d 8 +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=hgridshift +grids=ntv1_can.dat +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1 80.10096858 -70.89749190 0.00000000 inf $ echo "80.1 -70.9 0" | PROJ_LIB=/usr/share/proj src/cct -d 8 +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=hgridshift +grids=$HOME/proj/proj-datumgrid/north-america/ntv2_0.gsb +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1 80.10096782 -70.89749276 0.00000000 inf
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Seem okay to me. Better get a second pair of eyes before merging though. |
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When investigating the format of NTv1 and comparing PROJ code with the actual
header of ntv1_can.dat, I discovered that the longitude & latitude shift values
started at offset 192, whereas PROJ assumed that the header was 176 bytes only.
This caused PROJ to use the wrong offsets values (shift of one grid sample by
longitude). So the effect was moderately visible, especially on the latitude,
but when comparing with NTv2, one can see that the longitude value after the
fix seems to closer to NTv2.
old:
echo "60.5 -100.5 0" | PROJ_LIB=/usr/share/proj src/cct -d 8 +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=hgridshift +grids=ntv1_can.dat +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1
60.50022624 -100.50040292 0.00000000 inf
new:
echo "60.5 -100.5 0" | PROJ_LIB=/usr/share/proj src/cct -d 8 +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=hgridshift +grids=ntv1_can.dat +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1
60.50022403 -100.50041841 0.00000000 inf
echo "60.5 -100.5 0" | PROJ_LIB=/usr/share/proj src/cct -d 8 +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=hgridshift +grids=$HOME/proj/proj-datumgrid/north-america/ntv2_0.gsb +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1
60.50022348 -100.50041978 0.00000000 inf
old:
$ echo "80.1 -70.9 0" | PROJ_LIB=/usr/share/proj src/cct -d 8 +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=hgridshift +grids=ntv1_can.dat +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1
80.10096789 -70.89746834 0.00000000 inf
new:
$ echo "80.1 -70.9 0" | PROJ_LIB=/usr/share/proj src/cct -d 8 +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=hgridshift +grids=ntv1_can.dat +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1
80.10096858 -70.89749190 0.00000000 inf
$ echo "80.1 -70.9 0" | PROJ_LIB=/usr/share/proj src/cct -d 8 +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=hgridshift +grids=$HOME/proj/proj-datumgrid/north-america/ntv2_0.gsb +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1
80.10096782 -70.89749276 0.00000000 inf