Objective: Produce a multi-criteria model to assist in directing the search for potential habitats for the species, Spiranthes diluvialis, given a set of predetermined criteria for their preferred living environment: namely, low-slope, mid-elevation areas with a southern aspect approximately 725 hectares worth of total potential space. Construct digital terrain models (TINs, DEMs) to aid in constructing a geographic information surface to conduct the above MCE analysis.
“The Triangulated Irregular Network (TIN) model recreates a topographic surface by connecting measurable points of elevation with lines of slope and planes of aspect. A Digital Elevation Model (DEM) uses a raster, or cell-based, approach to represent topography” (UBC Geography, 2017).
Using Contours in GIS Analyses
Contours are not typically used in GIS analyses because they are not spatially continuous and therefore, can not easily interpolate the data: whereas GIS analyses rely on continuous iso-lines. For features like the unclosed lakes previously represented as line data, such would need to be converted into closed polygons by manual edge-matching. However, edge-matching assumes that the missing edge of the feature is perfectly straight and would thus decrease the accuracy of the feature representation.
Defining XY Tolerance
The XY Tolerance tool is used to group features with similar geometry, wherein the set XY tolerance value used is the shortest distance in between two clustered coordinate features: or rather, the minimum resolution of joining feature points to make lines and connecting line segments to create closed polygons. In this case, the XY tolerance (two meters) was used to connect line segments to create closed polygons in order to represent the lake features.
Setting too large an XY tolerance will over generalize the data by combining nearby lakes and showing them as one polygon. Setting it too small might not connect enough of the line segments to create a closed polygon of the lakes.
Digital Terrain Modeling Manipulation See attached map, ‘TIN Analysis of Southern Okanagan” for practice on digital terrain modeling manipulation.
TIN Analysis of Southern Okanagan
Selecting midpoints in Defining Areas with Southern Aspect
Selecting 180° (instead of 45°) as the midpoint assures that any aspect close to 180° degrees will have a southern aspect. When starting the degree measurement at 0° North, the convention is clockwise from North of the degree arc, such that 180° will always be perfectly South. Choosing 45° as the midpoint would define areas with a North eastern aspect.
Procedure
It was given that the preferred living conditions of the Spiranthes diluvialis, the area of interest would be approximately 725 hectares of low slope, mid elevation area with a southern aspect. The provided elevation, water, and transportation shape files were converted into feature classes to construct a TIN, and later a DEM, to be examined using a multi-criteria evaluation (MCE) model by the following procedures: factor identification, data normalization, and multi-criteria analysis. The ideal habitat factors, namely low slopes, mid elevation, and southern aspect, were normalized by establishing an ideal value within a range of reclassified values. Elevation was normalized around an ideal 750 meters, slopes around ‘low’ and aspect around 180° south. Values close to the ideal parameters were reclassified values close to 1 and otherwise, reclassified values closer to 0. The normalized data was tested by weighted importance in 2 MCE models to examine the spatial variability of assigning equal versus unequal weighted importance shown in the attached map, “Potential Habitats for Spiranthes diluvialis in Southern Okanagan.” weighted importance for the MCE models was calculated using the analytical hierarchal approach (AHP) calculator: see Fig. 1 and Table 1 below.
Figure 1. Equally and unequally weighted importance comparison using AHP calculator
Equally and Unequally Weighted Comparison Figure
Table 1. Equally and unequally weighted importance comparison using AHP calculator
Equally and Unequally Weighted Comparison Table
Multi-Criteria Evaluation Results
The first MCE model assumes equally weighted importance, assigning each factor 33% of importance. The map, “Potential Habitats for Spiranthes diluvialis in Southern Okanagan,” shows that 474 cells are potentially ideal habitats. The second MCE model shows an unequal weighted comparison wherein aspect (50%) is weighted greater than slope (25%) and elevation (25%), yielding 149 cells of potentially habitable area. This comparatively more restrictive weighted comparison would be helpful in cases when alternative habitats would need to be considered. The weighted sum for the MCE shows areas of overlap between infinite combinations of varying equally and unequally weighted comparisons: 2, 051 cells. This is important in considering how the spatial variability of potential habitats changes when varying the weighted importance. The suggested survey area is the overlapping region of the equally and unequally weighed importance, totaling 623 cells.
See attached map, “Potential Habitats for Spiranthes diluvialis in Southern Okanagan”
Potential Habitats of Spiranthes diluvialis in Southern Okanagan