Hello. I am one of the gmt users.
While using grdtrack, the -E option gave different results than expected, so I posted this question.
gmt project -C126/42 -E146/40 -G0.05 | gmt grdtrack -G@earth_relief_04m
The above code extracts tracks in units of 0.05 degrees, as expected.
126 42 0 581.55078125
126.06727303 41.9992195957 0.05 685.950447216
126.134544368 41.9983999151 0.1 778.567266292
126.201813932 41.9975409602 0.15 738.711357089
126.269081639 41.9966427332 0.2 786.50699371
126.336347405 41.9957052366 0.25 775.874512494
126.403611148 41.9947284728 0.3 651.538325028
126.470872785 41.9937124444 0.35 606.330429786
126.538132233 41.9926571539 0.4 583.099983292
126.605389409 41.9915626042 0.45 637.451147242
...
However, the following code…
gmt grdtrack -E126/42/146/40+d+g+i0.05 -G@earth_relief_04m -fg
differs slightly from 0.05 degrees.
126 42 0 581.55078125
126.066454721 41.9992293247 0.0494906569467 682.709060635
126.132922972 41.9984201334 0.0989926135381 780.37441656
126.19940458 41.9975724033 0.148505799718 738.998673986
126.265899368 41.9966861117 0.198030145332 781.983636776
126.332407163 41.9957612361 0.247565580126 781.71866822
126.398927787 41.9947977546 0.29711203375 661.76110162
126.465461066 41.9937956454 0.346669435755 603.199805729
126.532006823 41.9927548871 0.396237715597 589.333642864
126.598564884 41.9916754585 0.445816802634 622.493582344
...
The two profiles are almost similar, but there are subtle differences.
The plot above was drawn using the following code.
gmt begin profile png
gmt project -C126/42 -E146/40 -G0.05 | gmt grdtrack -G@earth_relief_04m >tmp1
gmt grdtrack -E126/42/146/40+d+g+i0.05 -G@earth_relief_04m -fg >tmp2
gmt plot tmp1 -R0/15/-8000/6000 -JX12c/3c -BWSrt -Bxa2f1 -Bx+lDistance -Bya4000 -By+l"Elevation / m" -W0.25p,red,solid -i2,3
gmt plot tmp2 -W0.25p,blue,solid -i2,3
gmt end
I’m using Ubuntu 22.04 LTS, gmt is version 6.3, and I installed it via apt install.
Why is there such a difference between the two commands? How to use grdtrack’s -E option correctly?
Thank you.