Geography 486 - Lesson 6
Representing Volumes and Surfaces - Week 2
Interpolated Precipitation Maps
Our lesson this week focused on creating shaded relief maps, generating isoline contours to denote varying elevation and rain levels, and creating maps of interpolated precipitation data for the state of Oregon.
In Part II of the lesson we were asked to create a couple of shaded relief maps of the state of Oregon showing different hillside shading and elevation contour settings.
For my first map I chose to keep the sun angle the same as the default value of 45 degrees as this provided the most visually appealing rendering of the terrain, but I moved the Azimuth to 290 degrees. I did this because there were several mountain ranges that fell in line with the default Azimuth of 315 degrees. Moving the sun's Azimuth allowed these features to be more clearly represented. I further enhanced this by upping the Z-Value to 10 from its default 1 value. This provided more contrast in the map to better see the terrain. This first shaded relief map (Figure 1) is shown below and uses the following specifications:
Altitude: 45 degrees
Azimuth: 290 degrees
Z-Value: 10
Isoline Contour Interval: 200 meters
Figure 1: Shaded Relief Map of Oregon, 200 Meter Elevation Contours
This map would be fairly difficult to read since the elevation contours are so tightly packed together. The same map with slightly different contour intervals is shown in Figure 2 below.
Altitude: 45 Degrees
Azimuth: 290 Degrees
Z Factor: 10
Isoline Contour Interval: 400 Meters
Figure 2: Shaded Relief Map of Oregon, 400 Meter Elevation Contours
These state-wide images are fairly jumbled and not very revealing. Figure 3 below is a closer view of an area in the Northwester part of Oregon, looking down on the Cascade mountain range. In this rendering, I used the same 400 meter elevation contours as above, but assigned a color ramp to the contour lines. The greens are the lower elevation contours gradually moving into the yellows for higher elevation contours.
Figure 3 Northwest Oregon Elevation Contours
I was a bit concerned that none of the orange or red contour lines were appearing in the Oregon map. However, if we take away the mask that we created to identify the state of Oregon, we can see that farther north the state of Washington contains some higher elevation contours. I guess the lesson here is that the contours are not tailored to the visible areas of the map, but is based upon the entire underlying grid, in this case the "hshade2904510" grid. Figure 4 below shows this.
Figure 4 Elevation Contours Without Oregon Mask
Finally, I was curious to see how the elevation contours related to the original Digital Elevation Map (DEM) from which the hillshade grids were calculated. Figure 5 shows the elevation contours laid out against the DEM.
Figure 5 Elevation Contours on DEM
In the example above (Figure 5) the isoline contours are clearly displayed against the different shadings in the Digital Elevation Map. Note the very high elevations identified at the top of the map (Mount Saint Helens?).
In Part III of our lesson, we were asked to utilize precipitation data gathered from a number of measuring stations located around the state of Oregon. We analyzed the amount of precipitation that fell on the state in the month of December 2003.
This next figure is a screen capture of the interpolated precipitation data for Oregon showing the surface classified into ranges symbolized by different colors. I used both the Inverse Distance Weighting (IDW) method and the Kriging method, but chose to display the IDW results. Figure 6 below shows the results of the interpolated data along with isoline contours representing levels of precipitation. For the interpolated data, I chose to go with a color ramp of brownish shades for lower levels of precipitation ranging to bluish levels for the higher levels of precipitation. I also modified the isoline contours, attempting to provide indications of the precipitation levels through both line size and line color. The black contour lines show the lowest precipitation contours, the green lines represent the moderate precipitation level contours, and the blue lines designate the highest precipitation levels. The major precipitation breaks are shown by the thicker lines at approximately 10 and 20 inches of precipitation. In addition to the interpolated surface area and the contour lines, the individual measuring stations represent varying levels of precipitation. The yellow dots indicate precipitation levels below approximately 10 inches, the green dots represent precipitation measurements of between 10 and 20 inches, and the blue dots are the measuring stations that reported more than 20 inches of precipitation.
Figure 6: Precipitation Level Interpolation Using the Inverse Distance Weighting Method
Keeping to the same theme, hillshading was added to the screen capture. In Figure 7 below, hillshading was added using the default hillshading values of 45 degree sun angle, 315 degree Azimuth, and a Z-Factor of 1. The hillshading is displayed with an 80% transparency value.
Figure 7: Precipitation Map with Default Hillshading
Finally, Figure 8 below displays the same coverage area but with different hillshading values. For this image, I used a 45 Degree sun angle, an Azimuth of 290 Degrees, and a Z-Value of 10. When I used the 80% transparency value, the hillshading tended to overpower the image, so this view is shown with an 85% transparency level.
Figure 8: Precipitation Map with Modified Hillshading
The following represents some of the data I have analyzed for the 2004 elections.
