Do all your shapefiles have to be a projected coordinate system in order to calculate geometry?

MelissaTraverso · ‎05-22-2017

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

MelissaTraverso · ‎05-24-2017

Is there a way I could reword this that would help get a response/tips/suggestions?

ChristopherMcClain · ‎05-24-2017

Mercator is probably not your best choice of Projections for calculating areas. You should look for something that is an equal area projection. Since you are only looking at Brazil there is probably some type of National projection that will meet your needs. You also need to identify what the Shapefile's actual projections are and define them before you do any re-projection.

MelissaTraverso · ‎05-31-2017

Hi Chris,

Can you explain what you mean by "You also need to identify what the Shapefile's actual projections are and define them before you do any re-projection." The shapefiles that I added in had an unknown projections. They appeared in the general vicinity of where I wanted them to be over Brazil so I projected them to GCS "WGS 1984".

How do I figure out what is an "equal area projection"? Do you mean choose a projected coordinate system that is most similar to GCS "WGS 1984"?

Can you confirm if it is possible to calculate geometry correctly if my shapefiles are GCS "WGS 1984" but I define my data frame to be a projected coordinate system?

MelissaTraverso · ‎05-31-2017

Would WGS 1984 Web Mercator Auxillary Sphere be okay to use as a projection in this case?

Thanks,

Melissa

ChristopherMcClain · ‎05-31-2017

Not a good choice.

Measuring distances and areas when your map uses the Mercator projection | ArcGIS Blog

ChristopherMcClain · ‎05-31-2017

You stated originally "Burn area shapefiles had an unknown projections so I projected all of them with WGS 1984." You need to determine what the coordinate system of the data is before you can project it into anything else. This can usually be done by checking the metadata or contacting the creator of the data. Once you identify what the Coordinate System is then you need to define the project for these shapefiles. then you can project it into the proper projection to do your analysis. I don't work with data from S America so I can't be sure but would assume your best bets are either "South American Albers Equal Area Conic" or "South American Lambert Conformal Conic".

MelissaTraverso · ‎05-31-2017

Hi Chris,

It looks like ArcMap recognizes the GCS as unknown but from the data source it sounds like a projection called "Plate-Carr´ee" was used. Do I know how to define the projection for these shapefiles as Plate Carree and then re project it to what I want.

From what I have read this morning it seems like "South American Albers Equal Area Conic" is a good choice for equal area projection. My next question is, in order for this to work I would need to project every shapefile to this projection? If I only set the data frame to this what are the consequences?

Thanks,

Melissa

RebeccaStrauch__GISP · ‎05-31-2017

The shapefiles that I added in had an unknown projections. They appeared in the general vicinity of where I wanted them to be over Brazil....

I'm assuming the "appeared in general vicinity" if in compring it to a basemap (raster or vector). You may want to check 1) the projection/reference-system of that basemap, and/or 2) for the dataframe. This may help you determine what you need to set you shapefiles to. As mentioned above, you don't want to project until you have the projection set for these.

-->One other thing to check, see if there is a .prj file associated with your shapefile. A few versions back, they tweaked the format of the names of projections in Desktop, so even if all the parameters were the same, it would not recognize the name. This happened to us. If you look at the .prj file with a text editor (notepad), you will see the name, and you should be able to find a match with the newer name. Assign this and it may work.

MelissaTraverso · ‎05-31-2017

Hi Rebecca,

When I look at the PRJ file this is what it reads

GEOGCS["unknown",DATUM["D_unknown",SPHEROID["unretrievable_using_WGS84",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]

but according to the user manual for these shapefiles it sounds like "Plate-Carr´ee" was used.

Thanks,

Melissa