I am developing an exercise on computing areas of polygons for a course in scientific computing that I teach. I'd like to have the students estimate the acreage burned in one of the fires in Washington state. I've downloaded a KML file which provides the perimeter of the current active fire zone, and can easily extract the lat/long coordinates used to describe the polygonal perimeter. Plotting the resulting points in Matlab confirms that I have the correct polygon as seen in Google Earth.
I compute the area enclosed by the perimeter polygon (using essentially polyarea in Matlab) to get an area in deg^2. To convert to sq km and then to acres, I calculate the length of the sides in km of a small box inside the perimeter. To get these lengths, I use an online calculator that converts distances in lat/long coordinates to km. Then I can scale the deg^2 by this scaling factor to get km^2 and then ultimately scale by 247 get acres.
The problem is, that the numbers I am getting are way off. For example, the Okanogan fire is reportedly 280,267 acres (from the same site as the KML data - http://inciweb.nwcg.gov) , but I only compute 146,516 acres. Calculations for other areas as just as far off. I am confident in the routine to compute the polygonal area, so am wondering what else in the above might be incorrect.
One possibility is that the reported "acres burned" are not flat acres (which is essentially what I am getting when computing the area as above) but take into account slopes of hills sides, complex terrain, etc.
Does anyone know if such factors are used to compute areas, particularly when reporting acreages burned? Are there any references as to what such factors might be?
Or is the problem something else entirely? I am not a GIS expert, so there could be something quite basic about how these areas are reported that I am missing.
أكثر...
I compute the area enclosed by the perimeter polygon (using essentially polyarea in Matlab) to get an area in deg^2. To convert to sq km and then to acres, I calculate the length of the sides in km of a small box inside the perimeter. To get these lengths, I use an online calculator that converts distances in lat/long coordinates to km. Then I can scale the deg^2 by this scaling factor to get km^2 and then ultimately scale by 247 get acres.
The problem is, that the numbers I am getting are way off. For example, the Okanogan fire is reportedly 280,267 acres (from the same site as the KML data - http://inciweb.nwcg.gov) , but I only compute 146,516 acres. Calculations for other areas as just as far off. I am confident in the routine to compute the polygonal area, so am wondering what else in the above might be incorrect.
One possibility is that the reported "acres burned" are not flat acres (which is essentially what I am getting when computing the area as above) but take into account slopes of hills sides, complex terrain, etc.
Does anyone know if such factors are used to compute areas, particularly when reporting acreages burned? Are there any references as to what such factors might be?
Or is the problem something else entirely? I am not a GIS expert, so there could be something quite basic about how these areas are reported that I am missing.
أكثر...