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NTv1 grid shift: fix file offset for reading of shift values in ntv1_can.dat #1144

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merged 1 commit into from Oct 11, 2018

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@rouault
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@rouault rouault commented Oct 8, 2018

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

…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
@kbevers
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@kbevers kbevers commented Oct 8, 2018

Seem okay to me. Better get a second pair of eyes before merging though.

@kbevers kbevers added this to the 6.0.0 milestone Oct 11, 2018
@kbevers kbevers merged commit 5e66226 into OSGeo:master Oct 11, 2018
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