When drawing x, y, -, + symbols, strange values appear in the postscript file

Q&A
1

Hello GMT users and developers. I’m posting this question because I had a problem with psconvert.
Running the following code will create a postscript file called figure1.ps.

gmt begin figure1 ps
    gmt plot -R0/10/0/10 -Jxy/1c -BWeSn -Sx0.5c <<EOF
2 2
4 4
6 6
8 8
EOF
gmt end

However, an unusually large value appears on line 721 of the figure1.ps file.

0 0 M
4724 0 D
0 4724 D
-4724 0 D
P
PSL_clip N
V
4 W
O1
236 945 945 Sx
405 60 7.09e+194 C
236 1890 1890 Sx
236 2835 2835 Sx
236 3780 3780 Sx
U
PSL_cliprestore
23 W
0 A
/PSL_slant_y 0 def /PSL_slant_x 0 def
2 setlinecap
N 0 4724 M 0 -4724 D S

“7.09e+194” is the number. It seems that ghostscript could not interpret this postscript file properly because of this value. If I delete only that line manually, there is no problem. This weird value appears only when drawing with x, y, +, - symbols. Does anyone know a solution to this problem?
My gmt version is 6.5, the operating system is Ubuntu 24.04 LTS, and gmt was installed via apt.

2

not reproducible, GMT 6.3 (Linux Mint 21) or 6.5 (same system, in a conda/mamba environment).

both generated identical PS code around line 721 using the provided code, with the faulty line not present there:

% PostScript produced by:
%@GMT: gmt plot -R0/10/0/10 -Jxy/1c -BWeSn -Sx0.5c
%@PROJ: xy 0.00000000 10.00000000 0.00000000 10.00000000 0.000 10.000 0.000 10.000 +xy
%%BeginObject PSL_Layer_1
0 setlinecap
0 setlinejoin
3.32550952342 setmiterlimit
0 A
clipsave
0 0 M
4724 0 D
0 4724 D
-4724 0 D
P
PSL_clip N
V
4 W
O1
118 945 945 Sx
118 1890 1890 Sx
118 2835 2835 Sx
118 3780 3780 Sx
U
PSL_cliprestore
23 W
0 A
/PSL_slant_y 0 def /PSL_slant_x 0 def
2 setlinecap
N 0 4724 M 0 -4724 D S
3

What is -Jxy? You mean -Jx?

4

I wrote the option in PROJ code
https://docs.generic-mapping-tools.org/6.2/cookbook/options.html#option-j

5

That’s the CookBook for the version 6.2 and I don’t think that option was ever meant for external use and probably is not used internally too, although it shows up in the Postscript comments. 6.5 does not mention it.

https://docs.generic-mapping-tools.org/6.5/reference/options.html#proj-codes

6

I was wrong. However, if I change -Jxy to -Jx, the same postscript file is still generated.

7

This is a known issue with GMT 6.5, and it likely happen on Windows only.

The issue has been fixed in Fix the bug for plotting stroke symbols by seisman · Pull Request #8410 · GenericMappingTools/gmt · GitHub but it not released yet.

8

Hi @seisman, @tllc46 reports Ubuntu as his operating system:

So apparently the problem isn’t Windows only.