Here is a brief analysis I conducted using ArcGIS of the forest cover over a sampled area adjacent to Willapa Bay. I used two orthoimages—arial photography adjusted for lens distortion and perspective effects—of the bay and surrounding area, taken in 2009 and 2011. The un-distorted quality of orthoimages makes them idea for analyzing land cover, as the areas of the photo should be accurate relative to each other all the way out to the edges of the photo. The hills surrounding Willapa Bay have a history of logging, and by drawing polygons over areas that appeared forested and areas that appeared to be recently cut, I was able to compare percentages of different types of land cover between the two years.
The analysis showed that the amount of cut area in the bay has increased (in fact, it has doubled as a percentage of the sampled area) from 2009 to 2011. This means that forest is being cut faster than it is regrowing. However, the total logged area is still small relative to the total forested area, so this does not immediately present a threat to the area’s forests, as long as the trend of increasing logging does not continue.
In addition, there may be bias in my selection of this area, considering that the sample was taken in order to perform an analysis of cover change. In this respect, the analysis could be improved by expanding the sampled area to the entirety of the terrestrial portion of the orthoimages.
It would also be useful, given a higher-resolution image, to be able to conduct this analysis with a greater level of detail concerning the age of the forest. With the current resolution, it is difficult to tell old trees from young trees, but having a measure of how recently the forest was logged would improve the specificity of the analysis. This would be especially useful if the comprise could be carried out over more than two time periods.
Overall, though, this was an excellent exercise in using GIS polygons to categorize and analyze areas of different land use, and I am a better analyst for it.
