dylan's blog

Comparing vector reprojection between GDAL 1.3.1 and ArcMap 9.0

Submitted by dylan on Sun, 2006-07-30 03:04.

it looks like the mysterious NAD_1927_To_NAD_1983_6 transform is actually a localized transform for the Quebec area. Furthermore, ESRI has informed us that this is not in any way a default transform, rather the first in the list. In summary, regardless of the software that you are using to do this type of work, always know your data and do your homework on the available methods. Thanks to Matt Wilkie for the detective work.

The Experiment
Quick comparison of the results of a vector projection using both OGR (GDAL wich uses the Proj4 library) and ArcGIS 9.0. Ideally both methods should return just about the same coordinates. The GDAL tools use the NADCON grid corection for datum shifts in the US and Canada, while ArcGIS provides several transformation methods (NADCON being one of them). Figure 1 demonstrates the dialog box where datum transformation parameters are set in ArcGIS. The default (first) method in this list NAD_1927_To_NAD_1983_6 is more than slightly confusing. Perhaps the rationale for this approach is explained somewhere, but I have certainly never come across it. In order to better understand any possible differences in the results of a datum shift operation, the three following operations were performed:

OGR ogr2ogr -s_srs 'prj=latlong datum=NAD83' -t_srs '+proj=utm +zone=11 +datum=NAD27' output.shp input.shp
ArcToolbox transform from NAD83 GCS to NAD27 UTM Zone 11 (NADCON)
ArcToolbox transform from NAD83 GCS to NAD27 UTM Zone 11 (NAD_1927_To_NAD_1983_6 transformation)


OGR vs. ArcMap 1: ArcToolbox projection dialog message
Fig 1: projection dialog in ArcMap
OGR vs. ArcMap 1: coordinate shift
Fig 2: observed coordinate shift

Navigating Wilderness Areas with GRASS (Where 2.0 Presentation)

Submitted by dylan on Wed, 2006-06-14 00:06.
Example DRG graphic
Figure 1: area of interest
Features extracted from DRG: lakes
Figure 2: lake features
Features extracted from DRG: trees
Figure 3: wooded areas
Example travel cost map
Figure 4: composite friction map
Graphical Example of least-cost path
Figure 5: least-cost path
vectorized trails
Figure 6: vectorized

PedLogic: An open-source, customizable pedon management system.

Submitted by dylan on Fri, 2006-02-03 04:14.

Converting GRASS vectors from 2D into 3D: v.drape

Submitted by dylan on Wed, 2005-09-28 23:12.

More often than not geographic data come in a format that is essentially two-dimensional: i.e. raster grids and vector points, lines, and areas with only (x,y) style coordinates.


Submitted by dylan on Mon, 2005-08-22 22:23.
Adding GRASS vectors to POVRAY Scenes: v.out.pov
GRASS Vectors
povray clipped ssurgo data
SSURGO cutaway
povray slope class cutaway image
Slope class cutaway
PINN Topo-map with POVRAY
McCabe Canyon and Temblor Formation
View of Mt. Defiance from Bear Valley
Near infrared
Over PINN: Geomorphic Features
Geomorphic features.
View of Mt. Defiance from Bear Valley. 2 meter true color
View of Mt. Defiance from Bear Valley, Pinnacles National Monument. Pan Chromatic 2m res.

Thanks to Markus Neteler for initial insight on how to build an appropriate POVRAY script file.

Geologic-Scale Erosion

Submitted by dylan on Thu, 2005-08-11 00:10.

Initial Landform

10 Iterations

100 iterations of mass removal based on preferential flow of water as calculated by r.topidx in GRASS. Notice how landform is cutdown most in stream channels, least at the ridges. The basins between ridges appear to "fill" with sediment near the 50th iteration as the entire landform is lowered to sea level (0m). Absolute change in elevation is visble in the elevation profiles below. Example GRASS commands below. Here is a link to a movie, containing all 100 iterations.

Transect Ideas: The Sierra Nevada Climo-biosequence

Submitted by dylan on Tue, 2005-07-19 21:31.

PDF Version here

Quick trip to the Ione formation

Submitted by dylan on Sun, 2005-06-26 19:20.
Fig 2: Road Cut
Ped features from the Ione soil.
Fig 1: Ped Features
Exposed surface
Fig 3: Exposed Surface
Landscape near the Ione Formation.
Fig 4: Landscape
Landscape on the Ione formation.
Fig 5: Another Landscape

There are few opportunities to see an Oxisol outside of the tropics. The Ione soil is one example of an Oxisol formed in a tropical paleoclimate, protected by a layer of ironstone, and later exposed during recent times. This soil is thought to have originally formed from highly weathered alluvium washed down from the original Sierra Nevada (Eocene age) and deposited in a low energy environment. Subsequent uplift coupled with repeated wetting and drying cycles transformed plinthite near the surface into ironstone. This extremely hard surface of iron stone protected the underlying material from erosion, and was eventually buried by cobbly alluvium of similar age to that of the China Hat formation. Later uplift of the Sierra Nevada and the resulting erosion of overlying material re-exposed the Ione formation materials. Recent colluvial deposits have created a considerable layer of overburden, masking the properties of the original Ione material near the surface. The subsurface of this polygenic soil contained oxic horizons: a horizon with less than 10% weatherable minerals in the sand fraction and a high content of low activity clays such as kaolinite (Buol et al., 2003). In addition, redoximorphic features such as iron concretions and nodules were found throughout the oxic horizons. Concentrations of hematite, goethite, and plinthite were also visible in the oxic horizons (See Figure 1). For the classification of Oxisols, an otherwise xeric soil moisture regime is recognized as ustic in the Keys To Soil Taxonomy 9th edition. This characteristic coupled with a base saturation greater than 35% is recognized at the great group level classification of this soil: an Eutroustox.