The Rosillos laccolith and surrounding bajada/pediment surface can be
analyzed for topographic information by using digital elevation data, or
DEMs.
DEMs may be organized in three basic structures for use in a geographic
or geologic information system to gather such information. Data may be
presented in raster square-grid format (most common), triangulated irregular
networks (TINs), or contour based data (vector-based). In this analysis,
the base data set used is a standard USGS raster grid DEM, available in
SDTS format via the Internet or directly from the USGS.
The USGS quadrangles which cover the area in and around the Rosillos Mountains (laccolith) are the Sombrero Peak quad, Grapevine Hills quad, Bone Spring quad and the Twin Peaks quad. The primary ares of concern (Cottonwood Wash and Star Creek fans, SRB) are found almost entirely on the Grapevine Hills and Sombrero Peak sheets. Using photogrammetric data capture and stereoscopic aerial photo interpratation, the USGS DEM's are available covering these quads and were imported for this terrain survey. The data covers 7.5' quads, at a resolution of 30m.
Several images in this analysis were created utilizing triangulated
irregular networks. These are created by creating a vector triang out of
every adjacent set of three raster points on the grid to produce a field
of irregular triangles. Aspect and 3D maps often employ these types of
structures.
Use of Topographic Attributes in Geomorphologic Analysis of the Rosillos Mountains
"Many of the most popular topographic attributes, such as slope, specific catchment area, aspect, and plan and profile curvature, can be derived from...elevation data for each and every element as a function of its surroundings (Wilson, Gallant 2000). Software programs of many varieties such as Mapfactory and Arcview use specific digital "tools" which process data either as a whole, in local groups or "neighborhoods", or by individual data points. Attributes which can be computed from DEM's which may highlight specific topographic characteristics for terrain analysis include the following:
Attribute Definition Significance
Altitude
Elevation
Climate, vegetation, PE
Upslope height
Mean height of upslope area
PE
Aspect
Slope Azimuth
Solar insulation, evaptranspiration, flora/fauna dist.
Slope
Gradient
overland/subsurface flow, runoff rate, precip.
vegetation, geomorph, soil water content, land class
Upslope Slope
Mean slope of upslope area
Runoff velocity
Dispersal Slope
Mean slope of dispersal area
Rate of soil drainage
Upslope Area
Catchment area above a short length of contour
Time of concentration
Dispersal Area
Area downslope from a short length of contour
Runoff volume, steady-state runoff rate
Catchment Area
Area draining to catchment outlet
Runoff volume
Specific catchment area Upslope area per unit width
of contour
Runoff volume, s-s runoff , geomorph
Flow Length Path
Maximum distance of water flow to a point in the catchment
Erosion rates, sediment yield, time of concentration
Upslope length
Mean length of flow paths to a point in catchment
Flow acceleration, erosion rates
Dispersal length
Distance from a point in the catchment to the outlet
Impedance of soil drainage
Catchment length
Distance from the highest point to outlet
Overland flow attenuation
Profile curvature
Slope profile curvature
Flow acceleration, erosion/deposition rate, geomorph
Plan curvature
Contour curvature
Converging/diverging flows, soil-water content,charc.
Tangential curvature
Plan curvature multiplied by slope
Provides alternative measure of local flow conv. and div.
Elevation percentile
Proportion of cells in a user-defined circle lower than the center cell
Relative landscape position, flora and fauna distribution.
abundance.
Summary of Results of Topographic Analysis of Cottonwood Wash Fan and Associated Alluvial Deposits
Altitude
Below is the standard unaltered form of the DEM grid, showing changes in elevation over area. The bright colored grids show areas of high altitude and the darker shades correspond to lower elevations.
Here you see a 3D representation of the same DEM, shown with the Z-values (height) for each cell projected into the vertical plane. Note the apparent "perspective" produced by this process. The gently sloping surface in front of the high points are the alluvial fans in question.
Below is a single quad DEM of Grapevine Hills which contain the specific fan deposits needed for study.
Upslope height can be derived directly from the normal DEM. A
histogram of data along the stream path gives a mean value of 824m
for the upslope area.
Aspect
Below is an aspect map derived from ArcView's Spatial analyst program. Each colored TIN represents a slope face which is oriented in one of the cardinal or subcardinal directions. The legend is presented for key. Note the southerly aspect present in the deposits south of the mountains. Most fan deposits are indeed sloping to the south here. Channels often show up as west-east slope pairs on the southern side of the Rosillos.
Slope
Slope models are creating by taking the first derivative of the DEM to derive change in elevation over a distance. The image below was created on ArcView Spatial Analyst, and shows the slope of the area as a series of values 0-90 degrees away from the horizontal. Note that most fan deposits range from .5 - 3 degrees in slope.
Upslope slope is acquired by histogramming the specific catchment basin and surrounding upslope areas feeding the fans. This value ranges from 82-17 degrees
Dispersal slope is the slope of the fan body itself below the apex. This ranges form to .
Upslope area is take to be the area of the catchment basin above a contour specified to mark the highest point of the actual feeder. The contour for this is set at
The Dispersal area of the fan is
Catchment area was taken as the combined upslope contributing area plus the specific catchment area, or the entire basin above the apex of the fans. This value for Cottonwood Wash is
The Specific catchment area, delimited by the short length of the contour, is .
Flow length path is the distance from any point in the catchment basin to a specified point. In this case, that point is at the apex. The image below shows distances to the apex in meters.
(insert flp)
The mean upslope length, dispersal length and catchment lengths, taken along steam pathways, were calculated to be respectively.
Profile curvature is derived from taking the second derivative of the normal DEM to find the change in slope over a distance along the fan surface. The image below is a grid which shows relative rates of slope change, or "Curvature" across the fan surface.
Plan curvature is found in similar fashion to the profile curvature, however, instead of finding the change in slope over distance, we are finding the change in aspect over distance.
The tangential curvature of the fan surface is found by multiplying aspect curvature with slope curvature to normalize the slope tendency across the fan surface.
The elevation percentile results are shown below. This gives
a percent of the area surface at or above a specific set of elevations.