Is QGIS a viable alternative to ArcGIS?
(Please note: This post is about clipping in ArcGIS version 10.0. The functionality has been improved, and problems mentioned have been fixed in later versions of ArcGIS)
I’ve never enjoyed working with contours. They seem to bog down my system more than any other layer type I work with. However, most of my clients are so used to looking at USGS Topo maps they expect to see them on at least one of the maps I produce for them. I recently worked on a project covering a five-town area in the Catskill Mountain region. The large area covered, and the ruggedness of the topography was proving exceptionally troublesome in processing their contours. So much so that I decided to look at other options to get the work done. I’ve used a variety of GIS tools over the years, but do most of my paying work exclusively in ArcGIS. It’s what I’m most familiar with, it does (nearly) everything I need it to do, and therefore provides my clients with the most efficient use of my time. However, in this situation that was not the case.
The one geoprocessing operation that frustrates me most often (in ArcGIS) is the Clip operation. It seems to take more time than most other geoprocessing tools, and often results in bad geometries. This happens so often, I usually resort to doing a union, and then deleting the unwanted areas of the Union results. For some reason this works much faster, and with more reliable results than doing a Clip.
Since what I wanted to do here was a clip on a contour layer, I was in for double trouble. Yes, I could have clipped the original DEM I wanted to produce the contours from first, then generated contours from the clipped DEM. But that wouldn’t have led to anything to write about. So, here’s a short comparison of how ArcGIS handled the process versus QGIS:
The hardware and software used:
ArcGIS 10, SP2
- Windows 7, 64 bit
- Dell Precision m2400 laptop
- Intel Core 2 Duo CPU, 3.06GHz
- 8 GB RAM
- Ubuntu 11.4
- Dell Inspiron 600m laptop
- Intel Pentium M CPU, 1.60 Ghz
- 1GB RAM
A fair fight?
I started out with ArcGIS, and loaded up my 20’ contour lines and a 1 mile buffer of the study area to which I wanted to clip them. I began the clip operation 3 times. The first two times I had to cancel it because it was taking too long, and I needed to get some real work done. Curious to see how long it would really take, I let the process run overnight. The progress bar kept chugging away “Clip…Clip…Clip…Clip…”, and the Geoprocessing results window kept updating me with its progress, so I assumed it would complete eventually. In the morning, I looked in the Geoprocessing results window and found it had run for over 12 hours before throwing an error, never completing the clip operation. The error message said something about a bad geometry in the output. Really, no surprise there.
(Yes, those are lines in the picture above, not polygons. They’re very densely packed)
QGIS gets to play
The next day I decided to give QGIS a shot at it. I copied the two shapefiles over to my 6 year old lappy. (The contour.shp file was 1.3GB) fired up QGIS, and ran the Clip operation on the two files.
This time it took all of 17 minutes and 21 seconds to get a new contour layer.
So, who’s the winner here? Was it a far contest?
My take-away is, ESRI really needs to do some work on its Clip geoprocessing tool. As I said earlier, it is slow, and results in bad geometries more often than any of their other geoprocessing tools I use.
Addendum June 11, 2011: See the follow-up post here: