Gmtspatial issue

Q&A
1

I’ve resurrected some old (GMT ver5) code that simply tested if a point (or points) was inside a polygon (poly.txt). The point gets written to a file (test.txt) and then:
gmtspatial <test.txt -Npoly.text+z
This produces zero output.

If I try:
gmtspatial <test.txt -Npoly.text+r
The response is:
Middle point from table 0 segment 0 is inside polygon # 999
even though there are six test points in the file (five of which are in the polygon).

What does it mean by “middle point”?
How can I get it to simply output the point plus the Z value?
The code worked previously but something has changed somewhere.

Thanks.

2

GMT 5 is deprecated - see GMT 6.4.0 as latest release. In the documentation for gmt select there is no +r or +z modifier to -N. Even GMT4 does not have that. In 6 I only find

Option -N...o is deprecated; use -E instead

but that is not your case.

3

Sorry Paul. I headed the post “gmtspatial” and then proceeded to type “gmtselect”.
I meant the former.

4

Can you post a MWE (reproducible example)?

5

Hi Joaquim.
Made up two files. One (test_poly.txt) a simple rectangular polygon and one (test_points.txt) a line of points crossing boundary of polygon.

test_poly.txt:
> -Z999
140 10
140 20
150 20
150 10

test_points.txt
138 15
139 15
140 15
141 15
142 15
143 15

gmtspatial <test_points.txt -Ntest_poly.txt+z
? (no response)

gmtspatial <test_points.txt -Ntest_poly.txt+r
gmtspatial: Processing input table data
Middle point from table 0 segment 0 is inside polygon # 999
gmtspatial: 1 segments found to be inside polygons, 0 were outside and skipped

I have since separated each point in “test_points.txt” with “>”. I get the same message (as above) for each point that is within the polygon, but still cannot get the -N+z option to produce any output.

6

It replies to me

C:\v>gmtspatial <test_points.txt -Ntest_poly.txt+z
138     15      999
139     15      999
140     15      999
141     15      999
142     15      999
143     15      999
138     15      999

but please don´t tell us that you are using GMT5

7

OK, I won’t tell you :slight_smile:

In the example, shouldn’t the 138/15 and 139/15 points be ignored (as outside)?

It used to work, but not anymore. Not sure why. I’ll klutz a workaround.

Thanks for your help.

8

I didn’t pay attention to the numbers (only if the replied). But indeed, on a quick check with the manual something does not look right.
For in-out tests like these I use gmtselect, and the result is different.

C:\v>gmtselect test_points.txt -Ftest_poly.txt
140     15
141     15
142     15
143     15
9

Thanks Joaquim,
Yeah, that worked for me as well, but I need to know which polygon the point is in so I can get a -Z value.

10

Does seem to have a bug. Will try to fix.

11

Fixed in master; see GitHub.

12

Are you following https://github.com/GenericMappingTools/gmt/pull/7747#event-10138661335?
Some more testing would be good.

13

Hi Joaquim,
Sorry for delay. Only work part-time so intermittent with web access.
Also, not computer literate enough to be able to compile and implement new code. Using windows executables.
Russ

14

You can try with this build that I just made
https://fct-gmt.ualg.pt/mirone/gmt-6.5.0_2023.09.01-win64.exe