GIS questions

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Amar Pai

Dec 15, 2009, 12:42:57 AM12/15/09
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Questions for all the GIS experts out there:

1. qGIS offers a choice of many formats importing data via "Add Layer." I noticed one of these is "CSV."   Does anybody know details of the csv format in gQIS?  Would I be able to export my bike graph to CSV and view it in GIS that way?  I'm imagining csv with rows of [Lat,Lon,Value1,Value2,...] for nodes and [StartLat,StartLon,EndLat,EndLon,Distance,Value1,Value2,...] for edges.

2. Another format qGIS supports is '.sql' file -- I think this is sqllite.  What would such a file look like and where would it come from? If I have a programmed sql file generated using "pgsql2shp" could I read that in directly some way? I'm guessing not, have to run it in a PostGIS DB and then connect to the DB.  Does anybody do this regularly now?

3. What is GRASS.  Do I need it? 



Matthew Heberger

Dec 15, 2009, 3:31:52 PM12/15/09

Usually, when I see CSV files in GIS, they are raster datasets. A raster is another name for a grid, and it is anything that can be represented by cells or pixels: aerial photos, elevations, land cover classification, etc.

We will mostly be interested in vector datasets, e.g. features that are points, polylines, or polygons.

Second part of your first question: If you want to bring in tabular data representing points that is in the form [Lat,Lon,Value1,Value2,...], I would do it using "Add XY data" in ArcMap. No idea how to do it in qGIS. Do you want to send me your file, and I'll convert it and send it back?

Update: I just looked at the qGIS manual. Section 12.3 describes the "Delimited Text Plugin". This does the same thing as the "Add XY data" function I described above: it pulls in a text file of points, and displays them on the map. I suspect this is read-only, and that if you edit the layer, you have to save to then export it to a different format. (?)

2. The qGIS manual says that sqlite files have the extension .sqlite. My guess is that the .sql file is a MySQL file. I manage my MySQL database with phpMyAdmin, and one of the export formats is .sql. It's a text file with a whole bunch of CREATE_TABLE and INSERT commands. They're nice, because you can use them to make backups, or make an exact copy of your database to a new location. I know that the new versions of MySQL have column types for holding spatial data.

3. I'm glad you asked about GRASS, because I just wrote a blog post with some information about the different open-source GIS programs that are out there.

GRASS is serious software, used by a lot of academics and researchers. It gives you access to LOTS of powerful functions. I would say, YES, we do need it, because it can create and edit topological data. When you're dealing with transportation networks, topology is critical. If you have no idea what I'm talking about:

qGIS is one of the UIs that you can use with GRASS... I think that the combination of GRASS, qGIS, and GDAL means that you will be able to do nearly anything you want, although it might take you a few months to figure it out.



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