Monday, February 6, 2012

4 Gigabyte GeoTIFFs

I had a fight with Arc 9.3 today.  It refused to load a 4 GB GeoTIFF that was provided to me.  It was a 1.0 m DEM.   It didn't even bother to try reading the raster into memory.  It simply failed with a generic error message.  Arc 10 on a different workstation could load it, however.   Drat.  I had to install Arc 10 today on my own workstation.  Good thing I can choose to create Arc 9.3 geodatabases with the appropriate tool.

Interestingly, Arc 9.3 is fine with massive GRID or Imagine rasters.  Sort of.  Actually, on my workstation, you really shouldn't bother.

In this case I needed a slope map.  I also needed 20 ft contours.  I didn't want to split the raster into pieces.  Clearly, for this project, the answer was to resample the data to a larger cell size.  This speeds up geoprocessing exponentially.  Just be sure to do it reasonably.

Tomorrow I will map out geomorphic features of a 2 or 5 m DEM using slope class among other things.  I love my job.

Sunday, February 5, 2012

Basics of Geologic Mapping using ArcInfo Workstation and ArcDesktop 10

ArcInfo Workstation has inherent topology.  As a Geologist, you can use it to ensure appropriate spatial integrity between rock units, faults, and contacts.  ArcDesktop does not, by default, empower users in this way.  That explains why educated GIS users can become very frustrated with those with no concept of GIS (how many slivers, overlaps, and self-intersections caused by "dangerous" GIS users have you had to fix?).  One must go through a series of steps to duplicate what Workstation provides at the get-go.

Fortunately, through the use of geodatabases and a few Arc tools, users of Desktop can ensure topologically correct geologic maps.  You must define topology for lines, generate "labels" using points, and then create areas (rock units) from these parts using the Feature to Polygon tool (Arc 10 no longer provides an option to generate polygons within the right-click feature dataset context menu).  If you're particularly savvy, you can export lines, points, and areas into a coverage for Workstation.  Unfortunately, Workstation will no longer be supported after Arc 10.  So... old-school geologists will need to move on.

If in the future you need to update only a part of the spatial data, you can clip, update lines, erase, load, generate polygons, and update (I may go into more detail later).

QGIS has yet to provide a neat way to enforce rules of topology.  GRASS, however, can do it without any problems.  It's basically built in as with ArcInfo Workstation.  But good luck convincing organizations to use GRASS -- even if those organizations had helped to develop it!

This post is really meant to grease the wheels and convince me to begin posting regularly again.  Will it work?