Re: [Qgis-user] Labelling expressions: how to select the 'last' row in a csv dataset
Bob, Thanks for the suggestion. How does one get the csv provider to read just the first line? Gavin On 10/03/2014 10:25 p.m., Basques, Bob (CI-StPaul) wrote: 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-boun...@lists.osgeo.org [mailto:qgis-user-boun...@lists.osgeo.org] On Behalf Of Gavin Macaulay Sent: Monday, March 10, 2014 3:18 PM To: qgis-user@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@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/qgis-user ___ Qgis-user mailing list Qgis-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/qgis-user
Re: [Qgis-user] Labelling expressions: how to select the 'last' row in a csv dataset
Brent, I was so focussed on trying to use labelling that I didn't think of other options, such as you suggested:) The two files method does the job, and matches the hacky way that the original file generated... I see some discussion of adding aggregate functions on top of QgsExpression [1], which as I understand it, would make such functions available for use in the labelling dialog. I could then do something elegant like datetime = max(datetime) to select the most recent datapoint... Regards Gavin [1] Towards the end of: https://groups.google.com/forum/#!topic/qgis-developer-remote/3pWsSyYSd9s On 11/03/2014 5:35 a.m., Brent Wood wrote: Hi Gavin, Can you have the row written to two files concurrently? One as an append one overwrite. Both watched in QGIS have the overwrite as the upper layer on the map with a different symbol, thus overwriting the trackline point in the append csv file. Not as elegant as an SQL solution, but csv's are not really reknowned for their elegance in data management :-) I haven't really thought this through, but you might try using a named pipe (see mkfifo on Linux, Windows has something along these lines too), run tail -n 1 -f myfile.csv into the named pipe, then open this in QGIS. This is such a horrible solution it's almost certain not to work, but you never know :-) Cheers, Brent *From:* Gavin Macaulay ga...@macaulay.co.nz *To:* qgis-user@lists.osgeo.org *Sent:* Tuesday, March 11, 2014 9:18 AM *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@lists.osgeo.org mailto:Qgis-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/qgis-user ___ Qgis-user mailing list Qgis-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/qgis-user
Re: [Qgis-user] Labelling expressions: how to select the 'last' row in a csv dataset
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-boun...@lists.osgeo.org [mailto:qgis-user-boun...@lists.osgeo.org] On Behalf Of Gavin Macaulay Sent: Monday, March 10, 2014 3:18 PM To: qgis-user@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@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/qgis-user ___ Qgis-user mailing list Qgis-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/qgis-user
Re: [Qgis-user] Labelling expressions: how to select the 'last' row in a csv dataset
Hi Gavin, Can you have the row written to two files concurrently? One as an append one overwrite. Both watched in QGIS have the overwrite as the upper layer on the map with a different symbol, thus overwriting the trackline point in the append csv file. Not as elegant as an SQL solution, but csv's are not really reknowned for their elegance in data management :-) I haven't really thought this through, but you might try using a named pipe (see mkfifo on Linux, Windows has something along these lines too), run tail -n 1 -f myfile.csv into the named pipe, then open this in QGIS. This is such a horrible solution it's almost certain not to work, but you never know :-) Cheers, Brent From: Gavin Macaulay ga...@macaulay.co.nz To: qgis-user@lists.osgeo.org Sent: Tuesday, March 11, 2014 9:18 AM 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@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/qgis-user___ Qgis-user mailing list Qgis-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/qgis-user
