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cpurvis
Joined: 24 Feb 2025
Posts: 255
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| Posted: Wed Aug 04, 2025 3:38 pm Post subject: example of how to render particle trajectory data (drifters) |
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Working demo is here: http://nautilus.baruch.sc.edu/~cpurvis/particle_demo/particle.phtml.
How to go from ASCII particle data to time-based maps.
Its original inspiration is from here: http://nautilus.baruch.sc.edu/rs.
This working example is based on PHP and MapScript w/ Mapserver 4.2 doing all the dirty work + PostGIS and PostgreSQL.
Step one is to get the data into the database.
The original data looks like this:
| Code: | 2025-05-10 17:40:00 | -5.666 | 43.544
2025-05-10 18:17:00 | -5.660 | 43.543
2025-05-10 19:18:00 | -5.661 | 43.545
2025-05-10 21:13:00 | -5.662 | 43.546
2025-05-10 22:06:00 | -5.667 | 43.544
2025-05-10 23:46:00 | -5.662 | 43.547
2025-05-11 02:11:00 | -5.664 | 43.543
2025-05-11 03:51:00 | -5.663 | 43.545
2025-05-11 04:53:00 | -5.695 | 43.611
2025-05-11 05:32:00 | -5.745 | 43.694
2025-05-11 05:53:00 | -5.724 | 43.727
2025-05-11 06:31:00 | -5.804 | 43.797
2025-05-11 07:32:00 | -5.850 | 43.904
2025-05-11 07:51:00 | -5.884 | 43.917
2025-05-11 09:12:00 | -5.869 | 43.909
2025-05-11 09:33:00 | -5.863 | 43.903 |
See particle_test.sql.txt for the actual INSERT and TABLE creation statements. This will create a new table, particle_test in an already existing myGISdb PostgreSQL w/ PostGIS database. (NOTE: You could also create a shapefile to do the same thing.)
The back-end is composed of two PHP pages. The first, particle.phtml, controls what time_stamp is passed to the map. It is pretty straightforward since it calls itself w/ the new (or no) time_stamp. It is attached as particle.phtml.txt.
The brains are mostly in the PHP file mk_particle_map.php (attached as mk_particle_map.php.txt). Of course, it depends heavily on the original .map running the show. It is attached as particle_test_map.txt.
The important part of the .map looks like this:
| Code: | LAYER
NAME "particle_trajectories_old"
STATUS ON
DATA "the_geom from particle_test"
TYPE POINT
CONNECTIONTYPE POSTGIS
CONNECTION "***"
TRANSPARENCY 20
CLASSITEM 'time_stamp'
CLASS
SYMBOL 'circle'
SIZE 5
COLOR 255 0 0
END
END
LAYER
NAME "particle_trajectories"
STATUS ON
DATA "the_geom from particle_test"
TYPE POINT
CONNECTIONTYPE POSTGIS
CONNECTION "***"
CLASSITEM 'time_stamp'
CLASS
SYMBOL 'circle'
SIZE 6
COLOR 255 0 0
END
CLASS
SYMBOL 'circle'
SIZE 5
COLOR 255 32 32
END
CLASS
SYMBOL 'circle'
SIZE 5
COLOR 255 64 64
END
CLASS
SYMBOL 'circle'
SIZE 5
COLOR 255 96 96
END
CLASS
SYMBOL 'circle'
SIZE 5
COLOR 255 128 128
END
END |
I'll hopefully have more time to go into more detail, but the essence is this:
* Two layers control the particle dots you see. The first (particle_trajectories_old) shows a transparentish red layer of dots for dots that are older than whatever time_stamp you're looking at. The second layer (particle_trajectories) shows different sized dots at varying shades of red to show (in our case) the most recent 5 hours relative to the time_stamp.
* What dots to show for a given time_stamp are controlled differently for the two layers. particle_trajectories_old is controlled by a FILTER statement since we want to show all the old dots relative to a time_stamp. Its WHERE clause might look something like this: | Code: | | date_trunc('hour',time_stamp) < date_trunc('hour',timestamp without time zone '2025-05-11 02:00:00') |
As you already know, the particle_trajectories is broken down into classes. Well, we use an EXPRESSION to control which class gets shaded a darker red and is a bigger dot. I assume that class0 is the biggest and reddest (and, hence, equal to time_stamp). If you look in the mk_particle_map.php, the line
| Code: | | $this_class->setexpression('/'.$time_stamp.'/'); |
does that for us. It does a regular expression on the results from the regular query to compare the yyyy/mm/dd hh part of the data w/ the computed time_stamp.
And there you have it!
Look at the working demo at the URL at the top of this page as well as a link from that page to another example of trajectories based on SEACOOS model data (it's an animation).
| Description: |
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particle_test_map.txt |
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particle.phtml.txt |
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| Description: |
| Only the CONNECTION string has been removed. |
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| Filename: |
mk_particle_map.php.txt |
| Filesize: |
1.29 KB |
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| Filename: |
particle_test.sql.txt |
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