Geographic vs. projected coordinate system

Question

Hi, I would need to deliver a result shapefile in WGS 84 (GCS: Geographic –lat/lon- coordinate system), the shapefile is originally in WGS84; but in the process of creating this result shapefile I need:

* To measure areas (the ArcGIS Calculate geometry tool doesn’t allow calculating areas with the Data frame coordinate reference system set to a GCS)
*  To give raster resolution in meters, for example in the Cell Size attribute of the Polygon to raster conversion tool. It must be entered in degree units, what means that I have to make a conversion between degrees and meters, which is not accurate.

Then, the corresponding solutions I find in ArcGIS, and associate doubts, are:

* set the Data Frame coordinate system to a proper projected coordinate system for that zone, 
* work with the shape layer, in WGS 84 (GCS) whereas the Data frame is in a Projected coordinate system,
* once finished working and obtained the result shape layer: Export the shapefile by doing Data > Export Data and , for the option “Use the same coordinate system as:”, choose “this layer’s source data”.

The doubts are these:

* Is it possible to make an accurate area measurement in a layer in meters, hectares, acres, etc, when the layer’s coordinate reference system is a GCS (though the Data frame has a projected coordinate reference system defined)?
* When doing in the result shapefile (in WGS 84) Data > Export Data and, for the option “Use the same coordinate system as:” choosing  “this layer’s source data”, is the resulting .shp going to be fully operative and to keep its geographic coordinates unmodified even if the Data frame coordinate reference system is a projected one?
* When creating a raster from a polygon, the raster is created in the projected coordinate reference system of the data frame. I need to deliver this raster (previously converted to polygon) in WGS84 (GCS); then, by doing the procedure

* Change the Data frame coordinate reference system to WGS84, and

* On the result raster, do Data > Export Data and, for the option “Use the same coordinate system as:”, choose “the data frame”,

is this an accurate way of dealing with that? I mean, aren’t there modifications in areas or coordinates of the delivered shape layer?

Thanks for any advice or idea.

robertsimpson · Answer

re 3rd question: I no longer trust/use "Use the same coordinate system as:”, choose “the data frame" to get to GCS WGS84 from a local Albers-NAD83.  I found it did not (re)project the TOC feature class at all. I now only use Model Builder and invoke the Project Tool in a Model Builder work flow.

MelitaKennedy · Answer

(Note: This is a late answer, probably due to discussions being moved around this week.)

It depends somewhat on which version you have. I think you're better off working in an equal area projection like Albers, customized to the area of interest. That allows you to calculate areas, use linear cell sizes, etc. Once you've done all the processing, use the Project and/or Project Raster tools to create data in WGS84.

Although, note this from the help at 10.2:

Python expressions can use the geometry area and length properties with an areal or linear unit to convert the value to a different unit of measure (for example, !shape.length@kilometers!). If the data is stored in a geographic coordinate system and a linear unit is supplied (for example, miles), the length will be calculated using a geodesic algorithm.   Using areal units on geographic data will yield questionable results as decimal degrees are not consistent across the globe.

I can't remember if Calculate Geometry has been updated for geodesic-based areas in 10.2, but the above can be done through Calculate Field.

On your second question/doubt, using the Export Data option and selecting 'use the data's coordinate system' will return a copy of the original feature class. You're correct on the procedure for your 3rd question too.

MelitaKennedy · Answer

Hi Chris,

A reference like that sends an email to me so it is a good way to contact me!