Assignment 4 – Least Cost Path Analysis

NR505 – GIS in Wildlife Sciences

Least-cost modeling is a method used for measuring the effective distance, rather than the Euclidian distance, between habitat patches. This method has been used in planning to assess the connectivity of existing or proposed reserves. GIS technology is an important tool for conducting least-cost analyses. Typically, a resistance surface in raster format, is the input to the least-cost modeling. This resistance surface is derived from one or more spatially explicit variables such as animal-habitat relationships, distance to development or other avoided areas, topography, physical barriers such as fences, roads, or streams. GIS habitat layers in polygon or grid format are weighted according to the expected resistance encountered by an organism when moving across the surface and linear features are then merged with the weighted resistance surface. Care must be taken when adding narrow features to the resistance surface to ensure that the narrow features are completely connected. ‘Cracks’ in narrow linear features may allow the least-cost path to incorrectly pass through an area with high resistance.

Recommended reading:

Theobald, D. M. 2005. A note on creating robust resistance surfaces for computing functional landscape connectivity. Ecology and Society 10 (2): r1. [online] URL: http://www.ecologyandsociety.org/vol10/iss2/resp1/

Rothley, K. 2005. Finding and Filling the “Cracks” in Resistance Surfaces for Least-cost Modeling. Ecology and Society 10(1): 4. [online] URL: http://www.ecologyandsociety.org/vol10/iss1/art4/

In the following exercise you will create a hypothetical resistance surface, add linear barriers, and compute the least cost path from a source location to other locations using the COSTDISTANCE and COSTPATH commands in ArcToolbox.

1)  Open a new ArcMap project and add the following data from the \Leastcost\ folder:

  1. Vegetation.lyr (this is a layer file connected to the grid vegetation in the same folder)
  2. Source_points.shp
  3. From_cell.shp
  4. Major_streams.shp (streams act as barriers in this exercise)

This landscape is composed of sagebrush and juniper woodlands of different structural stages. To exemplify the least cost path calculation over a resistance surface let’s consider the hypothetical organism the hypothetical sagemouse. The hypothetical sagemouse inhabits sagebrush habitats and can easily traverse areas with low canopy cover. It has never been found in young or mature juniper woodlands, aspen stands, curlleaf mountain-mahogany woodlands, mesic shrub lands or in very rocky areas. Perennial streams act as barriers for movement. The source points are known populations of the hypothetical sagemouse. The objective is to find the least-cost path from the three ‘from_cell’ points to the source points.

2) To create the resistance surface for hypothetical sagemouse movement, wildlife biologists have after much research and discussion agreed upon the following ranking of the habitat map. Your task is to reclassify the vegetation habitat types according to the ‘resistance’ score using the RECLASSIFY command in Spatial Analyst.

Value / Count / Cover / Description / Resistance
1 / 144516 / arar / Western juniper / Low sagebrush steppe / 0
2 / 45958 / arartab / Low sagebrush steppe / 0
3 / 265970 / artr / Mountain big sagebrush / 0
6 / 10469 / aspen / Aspen woodlands / 5
7 / 12751 / broadleaf / Broadlead riparian / 1
8 / 67165 / cele / Western juniper / Curlleaf mountain-mahogany / 4
9 / 26306 / meadow / Meadow / 0
10 / 37507 / shrub / Mountain shrub / 5
11 / 81453 / w1arar / Western juniper / Low sagebrush P1 / 0
12 / 51562 / w1artr / Western juniper / Mountain big sage P1 / 0
13 / 97291 / w2arar / Western juniper / Low sagebrush P2 / 2
14 / 82472 / w2artr / Western juniper / Mountain big sage P2 / 2
15 / 80708 / w4arar / Western juniper / Low sagebrush P3 / 4
16 / 104421 / w4artr / Western juniper / Mountain big sage P3 / 5
17 / 81894 / w5arar / Western juniper / Low sagebrush Mature / 4
18 / 146582 / w5artr / Western juniper / Mountain big sage Mature / 5
19 / 59874 / w2rock / Western juniper / Rock / 5
20 / 36491 / rock / Western juniper / Rock / 5

3) Perennial streams act as barriers for movement and it is therefire important to incorporate these linear features such that there are no ‘cracks’ in the resistance surface. In this step you will create a buffer around the perennial streams, convert the buffer to a raster dataset with the resistance value 1000.

·  Create a 30 m buffer around the streams to ensure that these linear features will be incorporated in the resistance surface without cracks. (ArcToolbox – Analysis tools – Proximity – Buffer)

·  Add a new field to the buffered streams and calculate this new field to 1000 (the resistance of a stream in the resistance surface). You can do this by opening the attribute table to the buffered streams – Options (lower bar) – Add new field. Name the new field Resistance, it is a short integer with precision 5. Right-click on the new field and calculate it to 1000.

·  Convert the buffered streams to raster data of 30 m pixel size (Spatial Analyst – Conversion – Feature to raster). Use the Resistance field as the value field. This will create a raster of the buffered streams with the value 1000.

·  Make sure you don’t have any cracks in your resistance surface!!

4) Merge the stream buffers with the reclassified vegetation. You can do this in the raster calculator.

·  Make sure you have defined the working directory, analysis extent and cell size under Spatial Analyst – Options.

·  Use the Raster calculator in Spatial Analyst to merge the two layers together. Activate Spatial Analyst – Raster Calculator. Type in the window

Surface = merge (‘double-click on streambuffer’, ‘double-click on reclass-veg’) Click Evaluate.

You now have a crack-free resistance surface where the habitats range from 0-5 and the streams have the resistance 1000.

5) In this step you will create the least accumulative cost distance from the resistance grid and the source point grid. The COSTDISTANCE command in ArcInfo or ArcToolbox calculates for each cell the least-accumulative-cost distance over a cost surface to a source cell or a set of source cells.

Open the Costdistance tool in ArcToolbox – Spaital

Analysis Tools – Distance – Costdistance

Input the name for the source_points.shp

the cost resistance surface

name the output grid (costgrid for example)

name the backlinkgrid

.


6) Finally you will calculate the least-cost distance from a location to any of the source cells.

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