How to get the nearest line from a point?

Q&A
1

Given a point (long and lat) within a country for example, how could I know which is the nearest line (like a coastline or international border)?

So basically I want to select the nearest line from a given point. The nearest line could be in all directions or a particular direction.

2

mapproject -L computes the closest distance from point to line. But the lines must be passed in an ASCII multi-segment file (but not in Julia and gmtmex, ofc)

3

Ok. Thanks.

And them how can I get the segment (from the multi-segment input). Should I use that distance and 'gmt select -L`, right?

4

Yes you would have to use gmt select -L but I don’t see how to do it (get the solution) without doing some programming/scripting. It turns out that this is not very different from the answer to this question (point in which polygon) in the Julia forum.

Basically one must search the minimum distance between a point and all lines in the multi-segment, and preferably use a first scan with BoundingBoxes to avoid the more expensive dist-to-line algorithm.

Now I leave it to the bash-lovers to solve it in the shell.

EDIT: Aren’t you in fact looking for the the point in which polygon problem?

5

Thanks Joaquim. I will have a look.

FYI, I am curious if it possible to create polygons (like DCW) from the GSHHG (or any other data set that replaces it).
My idea if to give GMT a point (like the centroid of a country), 2) search the nearest segments (in the coastline or international border) in all directions, 3) create a close polygon.

1 Like
6

Short answer is NO. Long answer is also NO. Very long answer is who knows, if with a lot of work. The problem is that GSHHG is organized by bins for performance sake, and those bins ignore administrative limits.

Same issue as above.

7

Agree with Joaquim on this. The information you need is not available in GSHHG or DCW. And if I remember right, even GADM does not have flags to tell if a segment is coastline or political - is that right?

8

AFAIK, all is political in GADM

9

Are you saying NO to my long-term idea? If want to get the closest line from another multi-segment data table?

10

I’m saying NO to the above.