2.1  Digital Elevation Model

To be able to meet the criteria of building on a slope of less than 25 percent, it was necessary to have a Digital Elevation Model (DEM).  There is a DEM of British Columbia with an accuracy of 20 meters, however this data is not free, and the purchase price exceeded the project budget of zero.  Therefore, the next best data set would be to collect data from the field.  This would involve using a GPS unit to collect elevation values at various locations on the property.  This was not possible due to time constraints and the lack of a GPS unit.  Therefore, the DEM was created from contour lines digitized from a 1:20,000 TRIM map.  This data set is the least desirable due to potential errors.

Potential Errors include;

• User Errors - Digitizing is labour intensive, therefore human errors occur.  To reduce the human fatigue issue digitizing was done early in the day when I was the most awake, as well as, digitizing section did not exceed one hour at each sitting.  In addition, ARCINFO was used for the digitizing process, since the user was familiar with the software.  This ensure that mistakes that might have been created from the lack of understanding the software where eliminated.  Human error was also reduced my using a map that was easy to read, so there was no confusion as to what needed to be digitized.
• Map Quality - The map used for digitizing was printed at a scale of 1:10,000 to increase the readablity of the map, as well as the map was not folded so the surface was smooth.

Sample Sites - When contour lines are used to create sample elevations there is the risk of creating a file with too many or too few samples, since all of the sample are clustered along the contour line.  This is not an effective sampling method.  To reduce possible errors when interpolating other elevations spot hieght were calculated and added to the vector file.  As well as, the command unsnapnone was used to remove unnecessary nodes from the digitized contour line.

Even with the above steps to remove potential errors, this data set will still have an accuracy less than the plus/minus 20 meters.  This is one of the reasons that the results can only be viewed as an initial selection.

Cartographic Model of DEM
Orthographic Perspective Display of DEM
 

This layer was created similarily to the DEM, in that it was digitized from a map.  The map was a forest cover map provided by Liz.  It was the only source of data on the type of vegetation on the property.  It would have been possible to use arieal photographs to create a map of the area, however the errors which would have been introduced range from inaccurate interpretation of vegetation boundaries to incorrectly calculation the scale.

Further error consideration;

• The map provided by Liz was a photocopy of an orginal map.  This presents problems with scale as well as distortion of the map itself.  Little can be done about the distortion, since the orginal map was not available, however a few steps were taken to improve the accuary of the data set.
• The orginal scale of the map was researched and all forest cover maps are created from a base scale of 1:20,000.
• The scale of the photocopy was calculated using the property boundary to determine that the map was not enlarged or shrinked.  The difference between the orginal map scale and the photocopied map scale was 500, therefore it was concluded that the map could be used for the project.  Again this contributed to the fact that this project is an initial selection process, instead of a final cabin selection.

Cartographic Model of Vegetation Classification
Map of Vegetaton Classification
 

The location of the Sandhill Crane nesting site was provided by Liz.  On a visit to the property,  Liz located the nest and recorded the UTM coordinates.  This was done use a handheld GPS unit, which would have a lower accuracy than a survey quality unit.  In addition, the accuracy of the signal was plus/minus 100 metres.  Therefore, heads up digitizing was used instead of generating a layer with the coordinates provided.  Liz did provide a discription of the location of the nesting site and this information was considered in the digitizing process.

Cartographic Model of Sandhill Crane Nesting Location

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