topic Re: Calculating Geometry of Data Collected in UTM in Coordinate Reference Systems Questions

Calculating Geometry of Data Collected in UTM

deleted-user-W17i9rKaOdiH — Wed, 06 May 2015 20:44:18 GMT

Hi,

If I have point data that were collected in UTM based on a datum other than WGS1984, can I use the calculate geometry function to get the point value in lat/long or do I first need to project the data and do a datum transformation before I calculate geometry?

I experimented with this by: 1. importing data collected in UTM based on WGS84; 2. creating two fields for the lat/long values; 3. setting the data frame to WGS84; 4. calculated geometry based on data frame with decimal degrees output versus first projecting the imported data to WGS84 before I do steps 1-4, and I get the exact same coordinate value.

I am wondering if this would be the case if the data were collected with UTM based on a local datum. Is it best practice to first project and transform the data before calculating geometry?

Any thoughts?

Thanks,

Carlynne

Re: Calculating Geometry of Data Collected in UTM

JoeBorgione — Wed, 06 May 2015 20:56:39 GMT

Can you identify the local datum from which to transform it? Unless you absolutely need sub-centimeter precision, I'd continue with your procedure...

Re: Calculating Geometry of Data Collected in UTM

ChrisDonohue__GISP — Wed, 06 May 2015 21:23:56 GMT

Sounds like something Melita Kennedy (ESRI staff/Projection-Coordinate System Guru (Guruess?)) could answer.

Chris Donohue, GISP

Re: Calculating Geometry of Data Collected in UTM

VinceAngelo — Wed, 06 May 2015 21:34:25 GMT

I've always described Melita as the "High-Priestess of Projection"

Re: Calculating Geometry of Data Collected in UTM

MelitaKennedy — Wed, 06 May 2015 22:21:09 GMT

Lyndy,

The only thing to change (if you're not doing it already) is to make sure you set a geographic/datum transformation in the map/data frame between the local datum and WGS84.

I just created some NAD27 UTM data and calc'ed the NAD27 latitude and longitude values and then set the data frame to WGS 1984 and calc'ed those. I set NAD_1927_To_WGS_1984_79_CONUS as the transformation. The results differed as I expected.

Melita