Can gmt read well-known text? Making movies with polygon plotting

Q&A
1

I want to create a movie using events plotting polygons.

The polygons have a start date and an end date. The polygons should appear at start date, and disappear at end date (so they have a duration). My data is delivered as a csv, with each row having a set of attributes and the geometry as well known text (https://en.wikipedia.org/wiki/Well-known_text_representation_of_geometry).

My hope was that I could use the same approach as I would with plotting points (i.e. row-by-row).

I don’t think it’s possible to read wkt with gmt, but if it is, how? Being able to process polygons on a row-by-row basis makes certain things easier.

I’ve looked at the events manpage (https://docs.generic-mapping-tools.org/dev/events.html) about plotting polygons, but it was not clear to my how to best proceed.

2

Hi Andreas,

I’d try and go the route via ogr2ogr with the layer creation option set to AS_XY (instead of wkt):

ogr2ogr -F OGR_GMT -lco GEOMETRY=AS_XY outfile.gmt infile.csv

This should then allow you to postprocess/read the file with GMT and do any magic you need for events. Depending on your workflow you should also be able to run SQL queries against your *.csv file during the conversion.

HTH,
Christian

3

Hi Christian,

Great tip. I’ll try this.

Thanks!

Andreas

4

Christian,

I ended up importing my data into an sqlite database.
I then do queries with ogr2ogr -F GMT -sql [..] to get what I want (changing dates in my case).
For each query I generate a new data file which is fed to gmt plot.
Pretty disk intensive I guess, but it works.

Thank you for the tip!

Andreas

5

Possibly, this would have worked too

using GMT

D = gmtread("that_CSV_file.csv", ogr=true);
1 Like
6

Julia? Some day, Joaquim. Some day.

Thanks!

7

Hi Andreas,

great ! I think the -sql option should also work with *.csv or *.shp files but SQLite of course also does the job. FWIW - the new Shapefile replacement Geopackage *.gpkg is a spatially-aware SQLite file (if you want to use your data with GIS like applications.

I guess similar to @Joaquim’s post, one could have done this in Python/PyGMT and geopandas, but as I assumed that you run events in a normal shell script and there ogr2ogr is likely the easiest option.

Cheers,
Christian

8

hi Joaquim,why I can’t install and use GMT.jl in Colab?

9

As I told you the other time I never used Colab. Why complicating? Just install in a normal machine.

10

Thanks again Christian.

I tried to find out what the difference is between ogr2ogr generated sqlite file and gpkg file (from a shapefile). I don’t quite understand it yet. It’s easy to get the geometry out of a gpkg file. I’m struggling to do the same with an sqlite file. I guess gpkg is the most convenient way to go as it is specifically made for spatial data.

I was also very pleased to see that you can access posgis functions like ST_Area, ST_Buffer etc. through SQL queries in ogr2ogr (via spatialite I guess). I did not know that this was possible.