2

The students in one of our labs have been cranking out daily satellite derived Sea Surface Temperature and Chlorophyll maps for a few years. They are published for public consumption and we have been using them in our global visualization lab overlaid on Google Earth for years.

We have been making them available on a lab website in the browser overlaid on Google Maps and as a Google Earth plugin. Now we would like to embed them into an ArcGIS Online web app so they can be more easily embedded on other lab pages.

The problem is that ArcGIS Online appears to be shifting the PNG overlay images, but a colleague of mine who has ArcGIS Desktop says that the images are not shifted when he opens the KML files using them using Desktop.

We opened a ticket with ESRI support and the answer we got back was:

"The reason for this discrepancy comes from the difference in projections between Google Earth and ArcGIS Online. Google Earth uses a "Simple Cylindrical projection with a WGS84 datum for its imagery base" [see reference: https://support.google.com/earth/answer/148110?hl=en] while ArcGIS Online uses WGS 1984 Web Mercator (Auxiliary Sphere). While these projections are similar, they're not the same, and therefore you will not see the exact same results in both maps when displaying the same data." And that "Although you are correct in that the data does not align in AGOL identical to Google Earth, the data is still be displayed as-expected, so I won't be able to log a bug or enhancement request for this situation."

While I imagine we could try to alter our code so that the PNG file is based on a different projection, I'm thinking that will hose those users who are referencing them for their research as now they'll have a problem displaying the KMLs using Google Earth. Plus I'm confused why ArcGIS Desktop would display them properly and ArcGIS Online would shift them.

The PNG files that the KMLs reference are updated daily and are available here: http://modata.ceoe.udel.edu/dev/mcimino/MODIS/GSFC/KMZ/ and http://modata.ceoe.udel.edu/dev/dhaulsee/MODIS/GSFC/KMZ/

Any thoughts as to how we can resolve this so that the existing users of the data aren't disrupted but where ArcGIS Online web apps can be leveraged to facilitate expanded web visualizations of the data? It seems naive if ArcGIS Online claims to display KMLs, but only if they're re-projected specifically for use on ArcGIS Online only.

Improve this question
5
  • What sort of shift are we talking about, 1-3feet, or 10-30metres?
    – KHibma
    Commented Feb 19, 2015 at 14:55
  • Kilometers - here is a test page that Ben spun up to test it: arcgis.com/home/webmap/…
    – cpuguru
    Commented Feb 19, 2015 at 15:08
  • We're trying to create a web app interface so that the users can select from various KML files for an ocean region - here's a test app that I tried for Antarctica, which also shows the shifting, but it's more difficult to tell without state boundaries and the like - ceoe.udel.edu/our-people/profiles/dw/orb-lab-antarctica
    – cpuguru
    Commented Feb 19, 2015 at 15:15
  • 1
    I'm trying to find more info, but this link doc.arcgis.com/en/arcgis-online/reference/kml.htm does state: "Image overlays that cover large areas may appear distorted."
    – KHibma
    Commented Feb 19, 2015 at 15:19
  • This is how we're currently serving up the KMLs - even Google Maps seems to play nice with them - orb.ceoe.udel.edu/maps/MARCOOS_SST/…
    – cpuguru
    Commented Feb 19, 2015 at 16:53

1 Answer 1

Reset to default
5

The coordinates of your bounding box seem to be at equivalent locations on "default" projected basemaps in Google Earth, ArcScene, ArcGIS Desktop, and ArcGIS Online. That makes sense based on my understanding: these projections are equivalent (EPSG:3857) and all use the WGS84 Datum (EPSG:4326).

Although we're dealing with KML ("Google") data, the coordinates are not in "Google" format -- the bounding box is plain old WGS84. Since the data was previously viewed on a 3D surface, it did not need to be projected. By default, Esri must project all data into EPSG:3857 when added to ArcGIS Online (since it must contain an Esri basemap, all projected in this system). I don't understand the Esri support response.

Perhaps it is indeed a bug or feature gap in projecting large extent KML GroundOverlay objects in ArcGIS Online as KHibma suggests. I have tested KML GroundOverlay to ArcGIS Online and it works correctly. It appears to be accurate up to +/- < 10 ft on smaller extents (tested up to ~100 x 100 miles) but have not be able to verify that KML GroundOverlay to ArcGIS Online projects accurately on large extent data(e.g., "quadrant", NE hemisphere). In the example data accuracy is around +/- 100 mile.

Maybe try breaking up your groundoverlay images into smaller extent tiles and referencing each in the KML?

Improve this answer
4
  • I just confirmed that it is the large extent issue as I noted above. Maybe it'll be enhanced in a future release. However, @bmeans answer here to break up the images into smaller pieces and have the 1 KML to combine them should put them properly on the map if you have the ability to chop them up.
    – KHibma
    Commented Feb 19, 2015 at 18:42
  • 1
    This has now been logged as a bug by Esri [ENH-000085856 Improve ArcGIS Online Map viewer to display large extent Image overlay KMLs without distortion.]
    – Ben Mearns
    Commented Mar 5, 2015 at 13:33
  • Apparently the ESRI dev team has rejected the enhancement bug fix that would allow large extent KML file overlays from displaying properly. They say it's a "known limitation" and won't fix.
    – cpuguru
    Commented Mar 11, 2015 at 11:53
  • Wondering if there's been any progress on this issue, as I'm still running into the problem when uploading a KML to ArcGIS Online. A 200 x 200 km raster at 1 km resolution doesn't strike me as a "large area", but that's subjective I guess.
    – David Roberts
    Commented Dec 18, 2017 at 21:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.