Open Source Computing and GIS in the UK

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ArcGIS Connectors for Postgis

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This post is a discussion of my experiences with a couple of ArcGIS connectors for Postgis. To me, a reliable Postgis connector would be so useful, as we simply don’t have the resources (or the inclination) to purchase the equivalent proprietary products. We are also looking for a solution that will provide a flexible backend for all sorts of interfaces, such as MapGuide Open Source, UDig, and so on, alongside ArcGIS.

The two connectors for ArcGIS that I have tried so far are Ziggis and PGarc. Neither of these are what I would call production-ready yet, and have some problems but they are being actively developed so there is hope!

Ziggis:

I spent a rather frustrating day yesterday trying to install the new release of Ziggis and thought it was worth blogging the steps I had to take to get it installed as I found information on this to be confusing and sparse. Digging around in the ESRI discussion forums and such like seems to suggest that my problems will be relevant to the subset of people who don’t have things like Visual Studio installed, and for all I know will not be relevant in ArcGIS 9.1 or later. The short version is as follows:

  1. Ziggis requires the Microsoft .net Framework and Arcobjects .net Assemblies to work,or the install will fail.

  2. To get the Arcobjects .net Assemblies, you must have Microsoft .net version 1.1 installed before you install ArcGIS and you must choose the custom install of ArcGIS Desktop (should be disk one).

  3. .net version 2.0 won’t work for ArcGIS 9.0 (don’t know about later versions).

  4. If you already have ArcGIS installed before you install .net then you will need to do a full uninstallation of ArcGIS, reboot, install .net, and then install ArcGIS. Trying to cut corners and running the repair/reinstall/modify option won’t work.

Once I had it installed, I had an extra button in ArcMap for adding layers from a postgis table. Ziggis using configuration files (.zig files) for providing the information about connecting to the database. There is an example file in the program folder that can be modified using a standard text editor. You add new layers by navigating to the appropriate zig file for the database you wish to connect to, then type the table name that you want to add. This is a slightly frustrating step because you have to know which table you want, ie by firing up some other Postgis/PostgreSQL administrator first. A method of browsing for available tables would be a bonus.

Unfortunately, so far I haven’t managed to get anything to display on the map! The layer appears in the table of contents, and I can open the attribute table, but the geometry column contains the value “no geometry” despite being able to display using a different connector (PGarc). The next step is to try and enable the debugging and logging options in Ziggis to establish the problem.

PGarc:

PGarc appears to be in a state of flux at the moment. Some development is ongoing as there are discussions on the postgis mailing list about it, but the website version has not been updated in some time. A few months ago I was given some updated code in the form of an ArcGIS project, and this version at least works for me- with an important proviso (more later).

PGarc installs itself as an extra toolbar with two buttons, one to add new data from a database, and one to save data back into a database. To connect to a database you need to create a system dsn with the connection details and provide the name of that file, along with your username and password to PGarc. PGarc does list the available tables, from which you can pick one and add it to your map.

Saving data back into a database seems to work, although it only saves the visible extent (so you have to zoom out to the extent of a layer before saving it).

My biggest problem with it is that if I have deleted a table in my PostgreSQL database, then PGarc cannot connect to it because it is still looking for the deleted table. I don’t yet know if this is related to the dsn, PGArc, or PostgreSQL.

Comparision:

Both projects are at an early stage and as you can see I have had problems with both of them. PGarc seems to have the best feature set at the moment (you can select features, which is not possible, afaik, with Ziggis) and save data back into the database. However, the idea of text-based configuration files is more convenient than a system dsn, as they can be easily transferred between computers. I need to investigate the geometry issue with Ziggis more thoroughly, and will keep following the discussions, and then I should be in a better position to comment.

There is at least one other connector available, but I didn’t get further than downloading the instructions! Furthermore, ESRI have intimated that Postgis connectivity will be available in a later release of ArcMap. This should be a good thing, but there are concerns as to how it will be implemented. Watch this space…

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