Sacramento Valley Digital Elevation Model Classification
by: Jesus Alvarez
ES 551 Computer Mapping Systems
Sacramento Valley's Topography
The Sacramento valley is located in northern California just northeast of San Francisco Bay. The valley's elongated basin is more than 200 kilometers long and flows north-south with the highest elevations just south of Lake Shasta and the lowest elevations at the delta. The valley boundaries are: the Coast Ranges (west), Shasta Lake area (north), Sierra Nevada (east), and the delta (south) (see figure 1. below).
The mountains surrounding the valley have undergone extensive erosion forming many of the v-shape valleys and creeks that flow into the great rivers that run across the valley toward the lower elevations into the delta. The valley's basin is filled with eroded materials from the surrounding mountains and marine sediments from past climate related sea level changes. The basin is relatively flat with exception of the Sutter Buttes, which are located about 100 kilometers north of the delta, with the highest elevation reading approximately 645 meters on the south facing Butte. The elevations in the lower basin range from below sea level at the wetlands and farmlands in the southern basin near the delta to just above 50 meters in the upper basin north of the Sutter Buttes.
Figure 1. Project location map. Boundaries: 37.8263888881, -120.763888889, 41.1083333328, -123.203888889 This map contains
a composite image mosaic from Lansat-7. 2002 Lansat data downloaded from CaSIL. Raster was imported and displayed using IDRISI.
Creating a DEM Display
The Sacramento valley has many distinct geographic features that can be appreciated if the terrain is graphically displayed with a Digital Elevation Model (DEM). However, building a graphic display of the terrain and showing the elevation changes in a relatively large, flat basin, surrounded by mountain ranges with steep slopes presented a challenge. The small changes in elevations in the elongated flat basin were difficult to show on a graphic display. In order to overcome the display challenge, GIS tools and cartographic techniques were used to model the DEM and show the small changes in elevations as well as the large changes.
For this project, 1 arc second (30 meter) resolution DEM rasters were downloaded from the USGS, The National Map Seamless Server. Covering more than three degrees of latitude this large geographic scale project presented another challenge since the raster files available in 1 arc second resolution covered less than two degrees of latitude. Therefore, a total of six DEM raster files were downloaded from USGS to cover the entire project area. Once downloaded, the raster files had to be cataloged into a single raster file that covered the entire project area to simplify the image processing. The new raster file was cataloged and converted to an ASCII file using ArcInfo spatial analysis tools and imported into IDRISI Andes using an import module to create the IDRISI raster file and DEM map display.
To enhance the DEM map display, the surface analysis module a "HILLSHADE" was used to create a hillshade raster that was blended with the DEM raster. In addition, a new quantitative symbol palette and annotation were created using the Symbol Workshop module in IDRISI to symbolize the DEM raster and to label the main geographic features of the project. The final enhanced DEM map display gave a better representation of all the distinct geographic features of the Sacramento valley (see figure 2. below).
Figure 2. DEM map. 1 Arc second (30m) resolution raster. Horizontal Datum NAD83, Vertical Datum NAVD 88.
This map uses a DEM raster, hillshade raster, and annotation layers created and displayed on IDRISI
From DEM to Classified Elevation Model
The DEM map shown in figure 2 clearly shows the ranges of the elevations from sea level at the Pacific Ocean to the high peaks in the mountain ranges to the north, however, the range of elevations is too large and the map display does not show clearly the elevation changes between the sea and the lower basin. In order to see the elevation changes in the lower basin and make the display more dramatic, the DEM raster needed to be classified using elevation classes with small ranges in the lower basin and large ranges in the steep slopes, for this task, the IDRISI RECLASS module was used.
The DEM raster was classified using small elevation intervals in the lower elevations to show the sea level and below sea level areas. Larger elevation intervals were used in the classes that covered the upper basin and steep slopes of the mountain ranges. The new classified raster was assigned a new qualitative color palette that was created in IDRISI to assign a color to each of the new elevation classes. A new map display was created in IDRISI; the new classified display clearly shows the sea level and below sea level areas as well as the distinct geographic features that outline the basin (see figure 3. below).
Figure 3. Classified Elevation Model. Classified raster created from the DEM raster using the values listed on the legend.
Rasters, annotation, color palette, and map display were created on IDRISI.
To get another perspective of the terrain the new classified raster was used to plot a 3D view. Using IDRISI ORTHO module, a three-dimensional orthographic perspective display of the classified elevation model was created. The classified image was used as the drape image and the original DEM raster was used as the surface image with a vertical exaggeration of 1.5x to make the 3D display more dramatic. The 3D display gives a better perspective of basin flow as well as the steep slopes, v-shape valleys, and Sutter Buttes that outline the valley and the sea level areas (see figure 4. below).
Figure 4. Ortho View of Classified Elevation Model. The classified raster is used as a drape image and DEM as surface image.
The view was created on IDRISI using ORTHO module with the parameters listed on the map
In most cases, the topography of a valley surrounded by mountain ranges can be modeled by graphically displaying a DEM raster with a quantitative color palette enhanced with a hillshade raster. However, Sacramento valley's topography presented a challenge in the lower basin. The changes in elevations in the elongated basin were small and the colors that symbolized the lower elevations were blending with the water features in the sea level areas near the Pacific Ocean. IDRISI image processing tools and cartographic techniques were used to help remedy the problem by classifying the raster using elevation intervals that allowed all distinct geographic features to be displayed with unique colors. In addition, a qualitative color palette and annotation layers were created in IDRISI's Symbol Workshop to enhance the graphic displays. The project was finalized by using IDRISI to create an orthographic rendering of the classified image using the classified raster with the custom qualitative color palette as a drape image and the DEM raster as the surface image with a vertical exaggeration for a more dramatic 3D display.
Maps Metadata: This project was created for ES 551 Computer Mapping Systems, Spring 2008. Project bounding coordinates: 37.8263888881, -120.763888889, 41.1083333328, -123.203888889, the project used (2004) 1 arc second resolution rasters from USGS and a (2002) lansat 7 composite image mosaic from CaSIL. The quality of the data used on the maps has not been verified, information and/or metadata about the data used can be found at the sites referenced above. All rasters were exported from the native GRID and geoTIFF formats to ASCII files using ArcInfo 9.2 spatial analysis tools before imported into IDRISI's raster format using IDRISI Andes, and are displayed using GCS, NAD83 Datum. All elevation units are metric and are rounded to the nearest meter using vertical datum NAVD 88. The DEM rasters were not corrected for sinks and negative values may include cells with missing data. Questions or comments about the maps and/or processes should be send to: firstname.lastname@example.org