An Exercise

DATA

This project uses data provided by the Urban Geology of the National Capital Area project. This collection of geoscientific information includes geological history, subsurface databases, stratigraphy, bedrock, surficial and hydrogeology maps and reports. Its stated purpose is "to provide engineers, planners, decision makers, and the general public with the geoscience information required for sound regional planning in densely populated areas." The study area covers an area of approximately 12,650 square kilometres (12 NTS 1:50,000 UTM 18n maps) centered around Ottawa. A number of coverages are provided, but the ones of concern for this project are: topography (topo.shp), lakes (lakes.shp), streams (streams.shp), surface geology (surfgeol.shp), hydrology (hydrogeol.shp) and, though used to a lesser extent, roads (roadsall.shp).

Individual coverages were opened in ArcView and examined for project applicability. The selected coverages were converted to shapefiles and cropped to an area of approximately 67 kilometres (east-west) by 57 kilometres (north-south). This area, centered around Ottawa, was selected because it encompassed site locations depicted in Ancient Ottawa: Site Inventory. One shapefile, lakes, was edited in such a way that the lakes and the rivers were divided into two different shapefiles. This was done to account for the different resources represented by lakes and rivers, which would have different effects on site location. The shapefiles were imported into IDRISI using the IDRISI import tool. Once in IDRISI, the following manipulations took place:

1. A field was added to the imported topo layer that indicated the elevation for each of the polygons. This field was exported (through the export avl values option in the database workshop) into a new values file. Through ASSIGN, a new vector map was created with the new values file. This map was then converted to a raster file (elevation) on which the CONTOUR module was performed. The resulting file,contour, was used to produce, using TIN and TINSURF, both a slope and an aspect coverage.

2. Similarily, a field was added to the streams, rivers and surfgeol imported files so that they could be mapped in IDRISI and so that the selected field of interest could be imported into a raster format (as opposed to the IDR_ID field). For example, the codes in the database that identified the surface geology types were in a text format. IDRISI is unable to map text string fields so the text codes had to be transfered to a numeric format.

   The newly created fields for each layer were then exported (through the export avl values option in the Database workshop) to create new value files. Through ASSIGN, new layers were created. All these layers were then converted to raster formats.

   The lakes database was futher edited to create a layer llake - lakes over 10,000 m square in size. This was to create a factor that was roughly similiar to 3rd, 4th and 5th order lakes, indicating a larger and potentially richer resource that would be more attractive for site location than that provided by smaller lakes.

   A field did not need to be added to hydrogeol as the code values for hydrology type were already numeric. This field was exported (again through the export avl values option) and through ASSIGN, a new vector layer was created with the new values file. As with the others, this layer was then converted to a raster format.

3. Using the information from Ancient Ottawa: Site Inventory, reference to 1:50,000 maps and the roads, rivers and stream layers, a site location layer was digitized and converted to a raster layer. Twenty four archaeological sites were located in the study area.

4. Lastly, an allwater layer was created by overlaying the lakes, rivers and stream raster layers (First covers second except where zero). This layer was then reclassed (0 to 1 and 1 to 0) to create an allwaterconstraint layer.