Batch projection of polygons to correct state lines lane zone?

[CaseyLott1]

Hello. I have a project where I need to make acreage calculations for hundreds of thousands of polygons spread across the U.S. I have 2 goals for this analysis: 1) minimize regional bias in acreage calculations, 2) generate acreage totals for many different subsets of polygons that are as close to true acreage as possible. These are my only goals, so my focus is on accurate area calculation only. In other words, I’m fine if the method I use distorts shape or distance and I don’t need to make any calculations across zones (if I stitch projections together). 

It seems to me like accuracy would be highest if the acreage of each polygon was calculated with it projected to the exact state plane zone it is within. Nice idea, but pragmatically, this seems like a nightmare.  In my fantasy world, there is already some sort of stitched together projection like this that covers the US where each location is projected to the correct state plane zone. I know this would wreak havoc around zone boundaries, it would look Frankensteinian, and cross-boundary inference would be nonsensical, but for my objectives, I think this would be ok. Does anyone know of an existing resource like this?

Alternatively, I wonder if there are any existing tools or workflows that could take a single feature class containing all of my polygons and somehow “batch project” them so that each individual polygon is projected to the appropriate state plane zone. Can anyone make recommendations about something like this? if anyone has a different solution to this problem entirely, I’m all ears.

Please note that I’ve already compared acreage measurements using Albers Equal Area Conic and Lambert Conformal conic with acreage estimates from the same polygons projected to UTM zones. Across this large dataset, both of the single projections had too many biased acreage calculations and the UTM based calculations had unacceptable amounts of inaccuracy for polygons that were farthest from central meridians. This is what led me to the idea of using state plane zones, since they cover smaller areas and have fewer problems with distortion. It’s just… there are so many of them that I’m finding this analysis quite daunting. I’m hoping someone here has some advice for me. I’m guessing that others have worked through this problem before. Thanks in advance for any sage advice you can provide.

[CaseyLott1]

The title of this post should read …”State Plane Zones”. I didn’t catch the error early enough to edit it.

[DavidPike]

https://pro.arcgis.com/en/pro-app/latest/tool-reference/data-management/calculate-geometry-attribute...

use the area geodesic option

[CaseyLott1]

Wow. Is it really this simple? If so, I have so many questions... starting with: why isn't the first thing we are taught when we start to learn GIS? Are all of the conversations I've ever had about which projection to use to measure area completely invalid? Why do people say: "Never use a Web Mercator projection to measure area; but use an equal-area projection instead".  Wouldn't it be more helpful if we said: "Never measure area using any kind of projection. Instead, use the geodesic area measurement option."

If geodesic area calculations report "true acreage" than why would we even need the option to calculate area from projected maps? If I'm understanding this correctly, then choosing the Calculate Area option instead of Calculate Area Geodesic option is ALWAYS a bad idea, just inviting inaccuracy. Also, does this mean that acreage calculations will be most to least accurate in the following order: 1) geodesic, 2) equal-area projections with small zones (e.g., state plane), 3) equal-area projections with larger zones (e.g., UTM zones and larger continental equal area projections); 4) any projection that is not equal area, and 5) web Mercator at the absolute worst? Again, why don't we all have t-shirts that say "always calculate area using geodesic methods"? I'm being a little cheeky here, but to just to be clear... is there any situation where one would NOT want to use a geodesic method to calculate area? 

Now that these general issues are out of the way, I'm still a little confused about how things actually work in ArcGIS Pro. For example, if I have a feature class that is delivered in Web Mercator, can I leave it in this projection and just use the Calculate Area (geodesic) option in the Calculate Geometry tool? I just tried doing this with feature classes that were in a few different projections and each time I got the same results from the geodesic calculation. Does this mean that I don't ever have to worry about projections at all as long as I use this Area (geodesic) option in the Calculate Geometry tool? Is there some situation where a feature class being in one projection, or a map being set to a different projection, will result in a different result if I calculate area using Calculate Area (geodescic) method?

In other words, do all of the demonstrations I've seen about distortions related to projections (which affect how I visualize the 3-D world in 2-D) have no effect at all on my ability to make accurate area measurements in ArcGIS Pro using the Calculate Geometry tool by selecting Area (geodesic)?

If this is true, then Hallelujah, I'll never worry about this issue again. If not, what else am I missing? 

Thank you David for your response and I'm interested to hear if things really are as simple as they seem.  

[DavidPike · Accepted solution]

I don't think I'm qualified to give you the answers to those questions!  However I would say that the geodesic methods haven't always been available as they can be computationally expensive.

I think localised projections e.g. state plane will always better fit the local geoid with their ellipsoid, and will always be more accurate.  there may be zones of the Earth where the geoid and WGS84 spheroid is out by a margin that is significant to calculations but I'm not sure of that either!  

if you're working in 1 state plane area then I'd certainly keep to planimetric area calculations, if you are working across multiple areas I think you take a small trade-off between the geoid-ellipsoid fit to make everything simpler.

I might recommend posting another separate question on here about this as I'm far from any authrority on this.