Higher Accuracy AK State Plane Projection?

[Anonymous User]

I am starting to see a newer Alaska State Plane projection in data I get from other GIS folks here in AK: NAD_1983_2011_StatePlane_Alaska_4_FIPS_5004_Feet

Is this a higher accuracy projection that is replacing the usually NAD_1983_StatePlane_Alaska_4_FIPS_5004_Feet?

What is the best transformation to use with this projection?

[RebeccaStrauch__GISP]

Tagging Joel Cusick​

[MelitaKennedy]

The projection portion isn't more accurate, but the geographic coordinate reference system (GCS, GeoCRS) is. The implication is that the latitude-longitude values that were projected to zone 4 (or if you unprojected the data back to lat-lon) are on the 2011 re-adjustment of NAD 1983.

A lot of data and its metadata/coordinate system information is confused. Many people use "NAD 1983" which should technically be used only for the 1986 realization for data that's really on a later re-adjustment like NSRS2007 or 2011 or a HARN re-adjustment.

In ArcGIS 10.4.0 (out soon), we're adding support for Geocon v1, which does have some transformations between HARN and NSRS2007 and NSRS2007 and 2011. We're hoping to get v2 into a later release.

Melita

[Anonymous User]

Melita,

That makes sense that it is the GCS (datum?) and not the projection that is now higher-accuracy.

What I am really trying to understand is what transformation should I use inside ArcMap so that everything plays nice together? I am still using ArcGIS 10.1 (we have plans to go directly to 10.4 once it has been out a little while).

The transformation options I am seeing are "composites" that use different combinations of WGS_1984(ITRF00)_To_NAD_1983_2011+ WGS_1984_ITR00_To_NAD_1983 etc.

There are about 4 different options in the GCS Coordinate System Transformation dialog.

Are there better or worse ones for Alaska? Or do all of them just tweak the datum a small amount that may not be more than sub-meter or sub-foot differences?

[MelitaKennedy]

Michael,

You can check the parameter values in the geographic_transformations.pdf file that's in the Documentation folder of your ArcGIS installation.

The combination you listed actually works out to a null transformation. Both use the same parameter values.

I often put in "equivalents." The parameters were originally for ITRF00 / WGS84 to NAD83 HARN or CORS96, for instance, but knowing that people have labeled their data with NAD83 instead, I used the same parameters. Sometimes it's also unclear whether the transformation (usually from the NGS CORS website) should be used for Alaska as well as the lower 48.

Melita

[JoelCusick]

Hi Mike,

Melita answered this a month ago. Not too late I hope.

Alaska is the same as CONUS on these transformations.  We also are in the same epoch of NAD83 as lower 48 in the eyes of NGS. Our geoid models are all Alaskan-grown and are getting quite good.  Only some datum transforms "transform" or "shift", or "tweak".  Melita mentions the null transform or what ESRI calls Bookkeeping transform. Those are found throughout the composite and singular shifter list. These are necessary at times to use.   A null transform doesn't move the data at all, merely re-defines.  Others do shift. Many shifts are very, very small, under decimter, some under 1 cm.

All the SDMI Spot /IFSAR data is tied to the NAD83 (CORS96) Epoch 2003.0 reference frame.  So, we in Alaska have been in modern variants of NAD83 for some time.

Your getting GIS data assigned with the NAD83 (2011) likely from those who are aligning GPS data to CORS and aligning them more specifically to the latest reference frame of NAD83.  It will pay in the future when the new realization or datum is rolled out in 2022 to very precisely defining the data we sit on.  This reference frame is going to be likely with us until around 2022.  Its only appropriate to apply shifters to/from NAD83 (2011) for data that really "is" in that frame. 

I tell folks to test these yourself.  Learn what the single transforms do, then build up into using composite transforms. 

Which version of NAD83 is assumed in geographic transformation?​