How do I know which PCS is most appropriate to project to?

[MelissaTraverso]

Hello!

I have a list of shapefiles that I am trying to merge together to create one larger shapefile. The issue here is that these shapefiles have different coordinate systems. The list below identifies all the different datums and coordinate systems I am working with. The top of each category in bold is the Datum followed by the italicized Geographic Coordinate system in that datum and the underlined Projected Coordinate System used with that particular datum and GCS. "NA" refers to no PCS used with that particular GCS in that Datum. Would anyone with expertise working with coordinate systems be able to suggest one PCS that would be best to batch project to?  I am most interested in polygon areas of these shapefiles so once I merge them I will need to calculate geometry to calculate the area of each polygon within the shapefile and I believe a PCS is necessary in order to do this. I am having trouble figuring out which PCS would be best given what I have below and if it is even possible to choose just one PCS given the different coordinate systems I have here. Any advice, info or tips would be great! 

Row Labels	Count of GCS	Count of PCS
D_SIRGAS	3	3
GCS_SIRGAS	3	3
SIRGAS_UTM_Zone_21S	1	1
SIRGAS_UTM_Zone_23S	2	2
D_SIRGAS_2000	7	7
GCS_SIRGAS 2000	7	7
NA	1	1
SIRGAS_2000_UTM_zone_22S	5	5
SIRGAS_UTM_Zone_22S	1	1
D_South_American_1969	1	1
GCS_South_American_1969	1	1
SAD_1969_UTM_Zone_22S	1	1
D_South_American_Datum_1969_96	1	1
GCS_SAD69(96)	1	1
SAD69_96_UTM_zone_23S	1	1
D_WGS_1984	48	48
GCS_WGS_1984	47	47
NA	32	32
WGS_1984_UTM_Zone_22S	11	11
WGS_1984_UTM_Zone_23S	4	4
LL84	1	1
NA	1	1
SA1969	14	14
LL-SA69	14	14
SAD_1969_UTM_Zone_22S	9	9
SAD_1969_UTM_Zone_23S	4	4
SIRGAS_2000_UTM_Zone_23S	1	1
SIRGAS	2	2
SIRGAS	2	2
SA_SIR_22S Transverse Mercator	1	1
SA-SIR-22S	1	1
User Defined	2	2
South_American_Datum_1969_/_Geographic	2	2
South_American_Datum_1969_/_UTM_zone_22S_(CM_51W)	2	2

Thanks,

Melissa 

[MelitaKennedy]

Since you're interested in calculating areas, you could reproject everything to an equal area projection like Albers. There is a continental South American Albers. Another possibility would be the Brazil Polyconic. Depending on what version you're on, there are ways to calculate geodesic-based areas even if the data is in a different coordinate system. 

Take a look at Add Geometry Attributes and Calculate Field help topics for information on calculating geodesic-based areas and lengths.

If, for other reasons, you want to stick with UTM, probably use UTM 22S, as you seem to have data in zones 21-23.

[MelissaTraverso]

Hi Melita, 

I ended up doing a batch project to South American Equal Area Conic listed under PCS>Continental. For the most past it looks okay but when I zoom into a polygon sometimes I notice that it overlaps slightly with a road where it shouldn't. It looks like it might be slightly skewed. Would you expect this to happen with any south american projection I re-project to simply because its a re-projection?

Thanks,

Melissa  

[MelitaKennedy]

Possibly--is the road very straight at that point? What could be happening is that the line should have a few more vertices in it. That is, the shape of the line should change with the reprojection, but isn't. 

Melita

[MelissaTraverso]

Hi Melita, 

I have noticed that all shapefiles that were a south american PCS or GCS are okay but from a few checks the shapefiles that just had a GCS WGS 1984 are all slightly skewed when batch projected to South American Equal Area Conic PCS. Is there a way to fix this or batch re-project? 

[MelitaKennedy]

They probably needed a geographic transformation. Do you know if one was set? I would try re-doing them but making sure that a transformation is set. You could check this by adding the SA equal area conic PCS data to ArcMap, add the WGS84 data and set a transformation. You should get a warning message.The warning dialog has a button that links directly to the transformation dialog. If you don't get the warning after adding the WGS84 data, open data frame properties, select the Coordinate Systems tab and click the Transformations button.