[GIS] Geographic Transformations in Alaska Between NAD83 and WGS84

arcgis-desktopcoordinate systemdatumnad83wgs84

I have done a ton of research on the most accurate transformation to use in mainland Alaska to convert between the NAD83 and WGS84 datums, but I just can't seem to find a definitive answer.

ArcMap gives 3 options for my area of interest:

WGS_1984_(ITRF00)_To_NAD_1983
- This one I am really unsure of and I have not found much information on when it should be used. I know it has something to do with epochs or realizations of datums, but I am in over my head on this one. Can someone please enlighten me?
NAD_1983_To_WGS_1984_5
- This is the one I think might be most applicable, but I am unable to find any definitive information to say whether or not it should be used for Alaska.
- Esri says it is applicable for Alaska, but I can't find confirmation of this anywhere else
- EPSG doesn't list this transformation when I search with coordinates in Alaska (or at all for that matter)
NAD_1983_To_WGS_1984_1
- I am fairly certain that this is a "null" transformation that doesn't actually do anything, and would therefore be least accurate. Especially in Alaska where everything shifts with time.

I should also note that we are using NAD83 (NSRS 2007) in our geodatabase and converting GPS survey data from WGS84 to NAD83 (NSRS 2007) with a custom transformation developed by a geodesist for our area of interest which shifts everything by a meter or so in x and y (the Esri transformation for this is a null transformation that doesn't actually do anything). What I am trying to do is load legacy/reference data that is currently in NAD83 by first transforming to WGS84 (my question here) and then transforming to NAD83 (NSRS 2007) via our custom transformation.

I need to define a definitive procedure for the project team to follow and it will be used on potentially hundreds of layers so I need to get it right.

Best Answer

WGS 1984 has had several "releases". I'm not sure whether or not to call them re-adjustments. WGS 1984 is loosely tied to the International Terrestrial Reference Frame (ITRF), maintained by IERS.

The first transformation, WGS_1984_(ITRF00)_To_NAD_1983, assumes that WGS 1984 is the one tied to ITRF00 and the NAD 1983 realization is CORS96 or similar. That version is also known as WGS 1984 (G1150). The current realization is WGS 1984 (G1674) which is related to ITRF08 instead. This transformation uses the non-time-related parameters from the NGS CORS website. They've since published a new transformation relating ITRF08 and NAD 1983 (2011), which Esri has as WGS_1984_(ITRF08)_To_NAD_1983_2011.

NGS has released, as beta, grid file-based transformations between "HARN" and NSRS2007, GEOCON, and between NSRS2007 and NAD 1983 (2011), GEOCON11. They do include Alaska. Esri has not incorporated them yet into the software. The GEOCON/GEOCON11 file format and interpolation method differs from previous NGS file formats and interpolation methods. Their beta version only accepts Blue Book-formatted data as well.

The 2nd transformation you list assumes WGS 1984 is ITRF96, so it's quite old compared to the other two. EPSG has either deprecated it, or lists the extent as excluding Alaska, but Esri has customers there using it which is why Esri lists it as covering Alaska too.

So for now, I"m not sure what to tell you. I think I would recommend WGS_1984_(ITRF00)_To_NAD_1983 as closest to NSRS2007.

Disclosure: I work for Esri.

Related Solutions

PROJ4 PostGIS Transformations – How to Convert Between WGS84 and NAD83 in Alaska

PROJ.4 uses the position vector method for its 7 parameter transformations. The ArcGIS transformation that you're referring to uses the coordinate frame method. They use different conventions for the rotation values, so switch the signs on the rotation values and retry.

General information:

To change the direction of a 7-parameter transformation (coordinate frame, position vector, Bursa-Wolf), change the signs of all parameters. This is not quite equivalent to calculating a full inversion of the rotation matrix, but usually fine within the accuracy of the transformation.

To convert between the coordinate frame and position vector methods, switch the signs on the rotation parameters only. The two methods use different conventions (one rotates the axes, the other the coordinates/vector).

So Esri's transformation is:

WGS84/ITRF00 to NAD 1983 (approx. CORS96 version)
method: coordinate frame
dx = 0.9956
dy = -1.9013
dz = -0.5215
rx = 0.025915
ry = 0.009426
rz = 0.0011599
ds = 0.00062

Change direction:

NAD 1983 to WGS84/ITRF00  
method: coordinate frame
dx = -0.9956
dy = 1.9013
dz = 0.5215
rx = -0.025915
ry = -0.009426
rz = -0.0011599
ds = -0.00062

Change method:

NAD 1983 to WGS84/ITRF00  
method: position vector
dx = -0.9956
dy = 1.9013
dz = 0.5215
rx = 0.025915
ry = 0.009426
rz = 0.0011599
ds = -0.00062

Note: I haven't tried it, but I think that's the issue.

Disclosure: I work for Esri.

Coordinate Conversion – How to Convert Between WGS84 and NAD83

Typically, NAD83 and WGS84 are within one meter of each other. Your concerns about differences of 2.5 feet, which are less than a meter, indicate you do need to perform this datum transformation.

Briefly, this calculation requires knowledge of when the coordinates were collected so that their movement over time can be accounted for (primarily due to the movements of underlying tectonic plates and partly due to "episodic motion associated with earthquakes"). You first convert the raw data to equivalent coordinates referenced to specific standard dates, such as 1997.0 or 2002.0. Then you look up 14 parameters describing (a) the offset, rotation, and rescaling (i.e., a seven-parameter transformation) needed to change datums at a given date together with (b) the first derivatives of those parameters with respect to time. Multiplying (b) by the elapsed time and adding to (a) gives a first-order approximation to the needed seven-parameter transformation, which you then apply.

Although the procedure is not mathematically difficult, it is rather involved and contingent on details of how your data were originally collected and processed. An NOAA paper, Transformations Between NAD83 and WGS84, provides a clear, readable explanation. It recommends you use interactive "Horizontal Time-Dependent Position" software, HTDP, available on the NOAA Website at http://www.ngs.noaa.gov/TOOLS/Htdp/Htdp.shtml. The Fortran source code consists primarily of the coefficients of the plate tectonic model plus some interface code, but I estimate around 3,000 lines actually do calculations. It could be a significant pain to port.

Best Answer

Related Solutions

PROJ4 PostGIS Transformations – How to Convert Between WGS84 and NAD83 in Alaska

Coordinate Conversion – How to Convert Between WGS84 and NAD83

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