topic Re: High Resolution Basemap Imagery in UTM NAD83 in Esri Training Questions

High Resolution Basemap Imagery in UTM NAD83

ArthurMorris — Tue, 23 Jul 2019 16:43:03 GMT

Questions about aerial imagery in ArcGIS Pro. I am analyzing vegetation patch shapes in small riparian corridors in the Rocky Mtns. UTM NAD83 with the appropriate zone works well for ha and distances, but available maps that I can project or transform correctly do not have fine enough resolution - e.g. NAIP 2018, 0.6 m resolution. It appears to me that basemap imagery in ArcGIS Pro cannot be reprojected or transformed to UTM NAD83. I am seeing that correctly? Are there any workarounds? Thanks.

Re: High Resolution Basemap Imagery in UTM NAD83

AdrianWelsh — Thu, 25 Jul 2019 20:31:28 GMT

Arthur,

In ArcGIS Pro‌, if your data frame of your map is in NAD83 UTM, then the Esri basemap will be reprojected on-the-fly to that projection. This has little to do with resolution. Can you describe better what you're seeing or not seeing? Maybe a screenshot?

You could also reach out to the surrounding agencies if they have local aerial data (cities, counties, states, and fed agencies).

Tagging Coordinate Reference Systems‌ and Imagery and Remote Sensing‌

Re: High Resolution Basemap Imagery in UTM NAD83

ArthurMorris — Mon, 29 Jul 2019 16:39:12 GMT

Thanks, Adrian, for your reply. I know it didn't have to do with resolution, but I wanted to explain why projecting the basemap would be helpful. After posting here, I shot an email to the Utah State University GIS lab, here is the pertinent part of their reply:

"I think your answer is in this blog post: https://www.esri.com/arcgis-blog/products/arcgis-enterprise/mapping/measuring-distances-and-areas-when-your-map-uses-the-mercator-projection/

In summary, you can digitize polygons from imagery in the Web Mercator projection, but as you mentioned, measurements/area calculations are not correct. Therefore, after you digitize your polygons from the basemap imagery, you will need to reproject those polygons to UTM NAD83 Zone 11N.

I did a quick test in ArcMap (not ArcGIS Pro). I created two ArcMap projects - one with the Web Mercator projection (WGS_1984_Web_Mercator_Auxillary_Sphere) and the other with the standard Utah UTM projection (UTM NAD83 Zone 12N). In both map projects, I used imagery that had been projected to two coordinate system of interest. I used AGRC basemap imagery (which is 2018 Google imagery) for the Web Mercator project and 2016 NAIP for the UTM project. In each project, I digitized an identical area in SLC and then calculated the area for both polygons. In the Web Mercator project, the area ended up being 3.549 sq km (prior to being reprojected) and in the UTM project, the exact same area was 2.042 sq km. So, clearly not the same!!! This suggests that Web Mercator is not good at providing accurate measurements.

However, when I reprojected the polygon that I created in the Web Mercator project to NAD83 UTM Zone 12N (and recalculated the area/geometry), the area was 2.036 sq km. The slight difference in the two values (2.042 vs 2.036) is attributed to my inability to digitize the area exactly the same. 😉

So, to answer your question, you can not reproject Esri basemap/Google imagery, but you can digitize data using basemap imagery. However, the key to getting more accurate measurements is to reproject your data when you are done and make sure you recalculate your area/geometry (i.e. update the area field in the attribute table)."

Re: High Resolution Basemap Imagery in UTM NAD83

AdrianWelsh — Thu, 08 Aug 2019 15:24:10 GMT

Arthur, thanks for the reply. I understand better now on what you're asking for. It sounds like your USU person should have used snapping, haha. Was it Chris G? Or Chris M that replied?