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NCAT vs ArcMap coordinate conversions

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02-22-2019 10:31 AM
JayJohnsonWashoeCounty
Regular Contributor

Our enterprise data (typical local gov datasets) is in NAD_1983_StatePlane_Nevada_West_FIPS_2703_Feet and datum=D_North_American_1983.  We have some new 3DEP lidar data that uses D_NAD_1983_2011, so I want to understand the differences between the two datums.

I've used the nifty NGS NCAT program to calculate the absolute XY shift between the two datums in our vicinity:NCAT coordinate conversion

My question is:  Why can't I get the same result in ArcMap?  

ArcMapProject

The transformation used above was automatically plugged in by ArcMap - I didn't select it.  Resulting coordinates in NAD1983_2011 are shown as X2 and Y2 below.

ArcMap projection coordinate result

One ASSUMPTION I've used is that ESRI's D_North_American_1983 means NAD1983(1986); that isn't explicitly stated anywhere I've seen.  What am I missing here?

Jay
Washoe County GIS
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MelitaKennedy
Esri Notable Contributor

NCAT is likely using NADCON 5 files which means it's stepping through these GCS:

NAD 1983 (1986) to HARN (1994)

HARN to FBN (2nd HARN, 1999)

FBN to NSRS2007

NSRS2007 to 2011

All four would be file-based transformations. Esri currently supports 2 steps only so we can't use the same path. Instead, we're using a 2000 era conversion between NAD83 (it's closer to HARN/CORS96) to "WGS 84" the a 2nd transformation to convert from "WGS 84" to NAD83 (2011). So they're not equivalent paths and won't give similar answers. We plan to add the NADCON 5 files, hopefully this year. 

You will probably get closer answers by using 

1st part

NAD_1983_To_HARN_Nevada

NAD_1983_HARN_To_NSRS2007_GEOCON_CONUS

2nd part

NAD_1983_NSRS2007_To_2011_GEOCON11_CONUS

This misses the conversion between HARN and FBN, unfortunately.

Melita

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2 Replies
MelitaKennedy
Esri Notable Contributor

NCAT is likely using NADCON 5 files which means it's stepping through these GCS:

NAD 1983 (1986) to HARN (1994)

HARN to FBN (2nd HARN, 1999)

FBN to NSRS2007

NSRS2007 to 2011

All four would be file-based transformations. Esri currently supports 2 steps only so we can't use the same path. Instead, we're using a 2000 era conversion between NAD83 (it's closer to HARN/CORS96) to "WGS 84" the a 2nd transformation to convert from "WGS 84" to NAD83 (2011). So they're not equivalent paths and won't give similar answers. We plan to add the NADCON 5 files, hopefully this year. 

You will probably get closer answers by using 

1st part

NAD_1983_To_HARN_Nevada

NAD_1983_HARN_To_NSRS2007_GEOCON_CONUS

2nd part

NAD_1983_NSRS2007_To_2011_GEOCON11_CONUS

This misses the conversion between HARN and FBN, unfortunately.

Melita

JayJohnsonWashoeCounty
Regular Contributor

Thanks Melita - excellent info.  I realize we are only talking a couple of inches at most, but I feel better at least understanding some of what is "behind the curtain".

Jay
Washoe County GIS
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