Monday, February 13, 2006

Open Source GIS on Linux

So I'm halfway into installing a desktop GIS on my home Linux machine. I started using PCLinuxOS as my main operating system at home a couple months ago, and am really enjoying it. It's the first Linux distribution I've tried that worked with all my hardware on the first try. There's quite a bit of software available on the repositories for free - almost anything you'd want for everyday home use, as well as a lot business apps.

My goal for now is to get Quantum GIS running and reading data from PostGIS. Since neither one is available in the PCLinuxOS repositories, I'll be installing both from source. My other option would be to find a linux distro that offers GIS functionality right out of the box, but most of those distros don't seem very up to date. Anyways, I'm looking at this as a learning expererience.

PostGIS installed easily enough. All the dependecies I needed were in the repositories, so it was just a matter of using Synaptic to install PostgreSQL and then following the instructions on the PostGIS website to install from source.

Quantum GIS has been a different story. One of its dependencies was not in the repositories, so I had to first install that from source. I had a little trouble with that, but got some quick help from one of the developers via the mailing list. Unfortunately, I had further troubles when it came time to install the actual QGIS package. In short, I can do ./configure, but when I try to make, I get a segmentation error. I've posted a question on the support forum, but so far no responses.

In related news, Mike at MJR42 is blogging about his own attempts to set up a similar GIS on Red Hat Linux. The main difference with his is that it will be GRASS and QGIS rather than PostGIS and QGIS. I'll keep watching his blog to see what happens.

Tuesday, January 17, 2006

US Survey Feet vs. International Feet

I get asked this now and then and I can never remember. This is not an issue for most people. If you live in the US, you are probably using US Survey Feet. If you're not in the US, you're most likely using meters. The only place I've ever seen International Feet used was Prince William County, who used that unit until about 2002, when they joined the rest of the country and started using US Survey Feet. Since that time, I've now and then run across a dataset that didn't get converted, or found a survey that's still in International Feet. In Virginia State Plane NAD83, you'll notice your data being offset by about 20 feet if you mix up the two units.

Here's a brief summary of how to convert from one unit to another:

1 International Foot = 0.3048 meters exactly
39.37 Inches (US Survey) = 1 meter exactly

To go from X US Survey Feet to Y International Feet:

Take X
Multiply times 12
Divide by 39.37
Divide by 0.3048 = Y
Overall conversion factor = 1.000002000004000008000016000032 (approximately)

To go from Y International Feet to X US Survey Feet:

Take Y
Multiply times 0.3048
Multiply times 39.37
Divide by 12 = X
Overall conversion factor = 0.999998 (exactly)
If you're in Autocad, you can scale all your features from 0,0 using the overall conversion factor. This assumes that whoever produced the drawing was using a coordinate system at all - a pretty big assumption at many firms.

If you're in ArcGIS, you can alter the projection properties so that the units are the correct one, and ArcGIS will do all the coordinate conversions on the fly for you.

Monday, January 16, 2006


Welcome to my blog. I'm Ben Slater, GIS Data Manager at Wetland Studies and Solutions, Inc. This blog is intended as a place for me to record various tricks and hacks I have learned while working as a GIS dude for the past 10 years and discuss whatever GIS related stuff I happen to be thinking about. My main interests are ESRI software (especially customizations), .NET programming, and open source GIS. I work in the environmental and archeology fields, though most of the stuff I do could probably be applied to any fields.

Some of the things I hope to be discussing are:

Fully automated map production in ESRI ArcGIS
Setting up a GIS in Linux
Methodologies for producing archeology maps
ArcGIS troubleshooting
General .NET programming and database programming

If anyone reads this and has some specific questions, feel free to leave comments or email me.