[Qgis-user] Labelling expressions: how to select the 'last' row in a csv dataset

[Basques, Bob (CI-StPaul)]

Gavin,

Set up the CSV file creation to append to the top of the file instead of the bottom, then just read the first line for the special cartography.

If you are running on Linux, there are a whole bunch of options with AWK/GREP, etc to get the data outputting in a more favorable fashion for reading.

My first thought was to suggest the SQL path though, this gives you lots of options for archiving and looking at the path/trail over time . . .

I've just completed doing something similar for a AVL tracking system.

Bobb



-----Original Message-----
From: qgis-user-bounces at lists.osgeo.org [mailto:qgis-user-bounces at lists.osgeo.org] On Behalf Of Gavin Macaulay
Sent: Monday, March 10, 2014 3:18 PM
To: qgis-user at lists.osgeo.org
Subject: [Qgis-user] Labelling expressions: how to select the 'last' row in a csv dataset

Hi,

I have a csv file that looks like this:

datetime, latitude, longitude, speed
2014-03-07T06:11:56.794000, 69.6930, 19.0321, 0.0 2014-03-07T06:12:56.791000, 69.6904, 19.0291, 10.0 2014-03-07T06:13:56.787000, 69.6879, 19.0257, 10.0

A new row gets added to the file every second or so. I load this file into QGIS as a csv file with the watch option turned on so that every time the map is refreshed, the map plots any new points that have been added to the file.

I would like to have the most recent line in the file plotted with a different symbol to the rest (the data represents realtime data from a moving ship so it's good to highlight the latest position). I can almost do this using the expression based labelling, but not quite (I can get a row number, but not the total number of rows, so can't do something obvious like: $rownum == $numrows, or $id == max($id)).

Any suggestions on how I can achieve this? I could move the data into a SQL-based layer and choose the last row using an SQL statement, but the simplicity of generating the csv files is attractive.

Thanks

Gavin
_______________________________________________
Qgis-user mailing list
Qgis-user at lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/qgis-user