Hi,
I am trying to calculate burn area of brazil using MODIS burn area product shapefiles for northern and central South America. These were monthly shapfiles from April - Decemeber of 2015. The goal was to merge/dissolve these shapefiles of burn area for this time frame and calculate the percentage burn out of total area of brazil. The area of the burn polygons were not included within the attributes so I had to add a field and calculate geometry to do this. The steps I went through are below. Perhaps I made a mistake somewhere because if you see the attached PDF you will see that my calculated percentage burned of 1.56% is a lot smaller than it appears in the imagery. Does the PDF look like 1.56% burned to you?
STEPS
1. Burn area shapefiles had an unknown projections so I projected all of them with WGS 1984.
2. I clipped the shapefiles to a shapefile of Brazil because the MODIS data was for northern and central south America.
3. For each shapefile I went to "add field", titlted in "Area" selected "double" and right clicked the field and went to "calculate geometry".
4. I ran into an issue here because my shapefiles were in a geographic coordinate system. I read online how to get around this so I made my data frame a projected coordinate system "WGS 1984 world Mercator". I was able to calculate area using the projected coordinate system of the data frame but keeping my shapefiles as GCS of WGS 1984 .
5. I then merged all monthly shapefiles together and then used the dissolve tool to merge multiple polygons into one polygon.
6. When I finally dissolved the merged monthly shapefiles so that the overlapping polygons would not double count the area my total area burned for brazil was 130,942 sq. km as opposed to 190,785 sq km from the merge. This was found by right clicking the attribute field "area" in the dissolved shapefile and selecting "statistics" and reading the number under "sum". The total area of brazil is 8,358,140 sq km. This gives you a percent of 1.56%
Is there something I could have done in this process to get the areas miscalculated? Potentially at step 4? Why does the imagery appear to be 20-30% burned but my calculation is much smaller?
Any helpful tips or advice would be appreciated!
Thanks,
M