Coordinate Systems for Web Maps and Hosted Feature Layers - Seeking Clarification

Question

Hi everyone,

I was under the impression that all Web Maps and hosted features layers must be in WGS 1984 Web Mercator (auxiliary sphere) to be published to ArcGIS Online. I was recently told that this is not, or at least may not be, the case.

I've looked around in this forum and elsewhere for a definitive answer on this, but I'm getting apparently conflicting answers - or, more likely, I'm misunderstanding the answers. So I wanted to seek some straightforward clarifications if possible.

1) If I create a map in ArcGIS Pro and then share that map to ArcGIS Online as a Web Map, am I required to create and share that map in WGS 1984 Web Mercator?

2) If I want to share something like a georeferenced raster or feature class as a hosted feature layer to be used in a Web Map on ArcGIS Online, does that hosted feature layer also have to be shared from ArcGIS Pro to ArcGIS Online in WGS 1984 Web Mercator?

I also have some follow-up questions that will depend on the answers to the questions above.

3) If the answer to question 1 above is "No," then is it possible to share a map from ArcGIS Pro to ArcGIS Online as a Web Map while using an Esri-provided basemap in something other than WGS 1984 Web Mercator? Is it the fact that I'm using an Esri-provided basemap that restricts me to sharing Web Maps in WGS 1984 Web Mercator?

4) If the answer to question 2 above is "Yes," then can this hosted feature layer be added directly to a Web Map on ArcGIS Online that is not in WGS 1984 Web Mercator, or is there an additional step required to make the hosted feature layer usable in a Web Map that is not in WGS 1984 Web Mercator?

Thanks in advance for your help. I've tried my best to keep my terminology as precise as possible, but sometimes discussing GIS issues feels like walking on a linguistic tightrope.

TrentonWalker · Answer

Thank you, this is all very helpful. I'm going to keep this thread "unsolved" for now to leave it open for others to contribute if they feel so inclined, but you've provided a lot of useful feedback and I really appreciate it. I need to look into a few things and run a few tests to see what kind of potential loss or inaccuracy we might be getting with our current setup before making any changes, but at least I know I have more options than I previously thought. Thanks again.

DavidSolari · Answer

Based on your workflow, you're either creating the hosted layers in Web Mercator or [State Plane]. I don't do a ton of work with hosted layers so I can't say what Pro is doing during publishing, you'd have to double-check the REST endpoints of the hosted layers after publishing.

When I said "online editing" I meant any process that goes through the web map. These edits will be entered as Web Mercator coordinates and then projected and transformed as needed to align with the hosted layer's coordinate system. Edits directly to the hosted layers are possible but if you're using Experience Builder, Field Maps etc. the web map is in between.

The loss from transformation differs on your location, you're best off loading a source feature class into pro, then using the Project tool to convert that to Web Mercator using the best transformation possible, then comparing both points. Transformation losses can accumulate over time but as long as your online edits aren't zipping features across the state or moving them dozens of times per day then you can usually estimate the loss by performing 2 or 3 Project tool passes back and forth from [State Plane] to Web Mercator.

If the losses are unacceptable then you'll need to assemble some base data and publish out a basemap in your state plane, then use that in all your maps. Note that the basemap doesn't need all state plane inputs, the transformation loss from generating tiles should be well within heads-up digitization tolerances.

TrentonWalker · Answer

@DavidSolari Thank you very much for the detailed response. It's extremely helpful. If you don't mind, I'll ask just a few more questions to make sure I understand the implications of your response.

For context, my organization works within a single U.S. state. I use a template file geodatabase (with lots of empty point, line, and polygon feature classes) to create Web Maps that we use in ArcGIS Field Maps to collect data in the field. The feature classes in the template geodatabase are all set to NAD 1983 (2011) StatePlane [my state]. This is because I need to make accurate length and area measurements in the field, and WGS 1984 is not suitable for this.

When creating a map, I open a new map in ArcGIS Pro, which includes an Esri-provided basemap by default, and then drag and drop the feature classes from the template geodatabase into the map. To publish as a Web Map, I have to change the map to WGS 1984 Web Mercator - but not each individual feature class.

My understanding is that while the map itself is WGS 1984 Web Mercator, the feature classes are still in NAD 1983, even when they're included as feature layers in the Web Map. If I set up an Arcade script to get certain fields to auto-calculate (such as length or area), I had assumed that the values provided would be in the default units of NAD 1983 and that all (re)projections are happening on-the-fly or have already occurred.

Based on your response, the potential accuracy and speed loss due to the datum transformation process is addressed - the datum transformation "price" is paid - either when I add my NAD 1983 feature classes to my WGS 1984 map in ArcGIS Pro or when I share as a Web Map from ArcGIS Pro to ArcGIS Online. My NAD 1983 feature layers are reprojected to WGS 1984 to match the Esri-provided basemap. The feature layers aren't turned into WGS 1984 layers - that is, the default units and state-specific accuracy for length and area measurements are the same as NAD 1983 - but the features are "adjusted" to display in the correct locations in a WGS 1984 map. This is my understanding of projection or reprojection. It may not be correct.

Is this a bad way to go about making a Web Map? Are there errors introduced at some point in this process that I should avoid by making a custom NAD 1983 (2011) StatePlane [my state] basemap and publishing our Web Maps in NAD 1983? The convenience of using an Esri-provided basemap, along with lots of Esri Living Atlas layers, is hard to overstate. It would be a significant burden to create a custom basemap for each Web Map I make (roughly one per week or 50-60+ per year), although I'll do it if it's required to get accurate measurements in the field.

One last question: when you say "online editing," what exactly does that mean? Does that mean editing in ArcGIS Online? Or would adding or editing features on a Web Map in ArcGIS Field Maps count as "online editing"? (For the record, we download offline areas for our Field Maps and synchronize them when we return to the office, but that might still count as "online" editing for this discussion.)