SWARS Spatial Analysis, The Book (4)

Select, Rank, and Weigh The Input Layers...


Now let's talk cooking!!

Of course that was a joke! However, similarities are there between cooking and our spatial analysis for SWARS. When cook, you don't just throw in all the ingredients you can find in the kitchen or use whatever quantities as you like. You choose the ingredients carefully and measure the quantities with perhaps even more caution, all depending on the dish you are trying to make. If you are making a Kung Pao Chicken, you wouldn't dump in 5 lbs of beef, would you? The same applies to the SWARS spatial analysis or any other spatial analysis for that matter. Input data layers are to be deliberately selected. For a weighted overlay modeling like SWARS, determining the layer rankings and weights requires much discussion and debate. And this, is where the participation of all your partners, stakeholders, and experts in the specific area really comes in.

We spent a lot of time during our Hawaii SWARS Workshop practicing how to conduct such a discussion session. The end goal is to find the right input layers and their relative significance (weight) for analyzing the issue in question. More layers is not necessarily always better. Selecting the right input combination, however, is. Please don't be tempted to put in all the stuff you got, for this is not a showing off of how abundant your data collection is.

For each layer selected, the "why" has to be asked! Why does it make sense to include that particular layer? What unique information does it contribute to the issue? How important is this data relative to the others? Where does it stand in the full selection of layers? What is the appropriate weight should it have? And then, how should it be used? All questions might not be easy but have to be answered.

Once again, let's resort to the Texas Report. This is how it describes the layer selection for the issue "Reduce the impacts of land-use change, fragmentation, and urbanization on forest landscapes":

...The Development Level layer received the greatest weight (40%) because housing density changes represent the primary force of change acting on forestland (20%). The forest patch layer is included (20%) to add priority for forest tracts larger than 500 acres, since the effect of development on these lands is fragmentation. Protected Areas, or Proximity to Public Land, confers a measure of priority (10%) because private lands between city boundaries and protected public lands could provide "green infrastructure" connections between natural areas and nearby cities...
*source: Texas Satewide Assessment of Forest Resources, Draft, September 26, 2008. p79

You, as the GIS specialist, not only should be present at these discussions, but also need to be actively participating. For the "non-GIS" experts to be fully mobilized, you have to help them understand the basic spatial analysis process. For example, a point source data can be easily buffered to create areal representations thus to be incorporated into the "overlay" modeling. Disagreement is likely to be inevitable but that is quite OK. Compromise shall be reached with all voices being considered.