CS-Map RFC # - NSRS2007 / NSRS 2011 Implementation

This page contains an change request (RFC) for the CS-Map Open Source project. More CS-Map RFCs can be found on the RFCs page. The change described in this request is to add support for the relatively new National Spatial Reference Systems of 2007 and 2011. These are, essentially, horizontal and vertical geodetic reference systems for the United States and its territories; produce by the National Geodetic Service of the United states. The acronyms NSRS2007 and NSRS2011 are commonly used to refer to these systems. They are also known as NAD83(2007) and NAD83(2011). Status

RFC Template Version	(1.0)
Submission Date	18 February 2014
Last Modified	18February 2014
Author	Norm Olsen
RFC Status	Draft
Implementation Status	Developed, under test
Proposed Milestone	Don't know who/how milestones are assigned; possibly 13.30(?)
Assigned PSC guide(s)	Norm Olsen(?)
Voting History
+1	Norm
+0
-0
-1
no vote

Overview While National Spatial Reference System 2007 )NSRS2007) has been around for several years, the shift defined by the new system, relative to the previous system (NAD83/96, aka HARN, HPGN, NAD83/91; we'll use HARN in this RFC document) was considered to small to deserve a defined shift definition. That is, the shifts were on the order of a few centimeters and at that time this was considered to be as small as the level of error. Fats forward to 2013, and precise and definitive geodetic shift models have been developed for NSRS2007. This was done at the time the US National Geodetic Survey was defining the National Spatial Reference System of 2011. Thus, at the current time, definitive models and algorithms exist for the migration of geodetic coordinates from HARN to NSRS2007 and subsequently to NSRS 2011.

Motivation

Please note that this is a sequential conversion process. That is, geodetic coordinates referenced to NAD83 must first be converted to HARN, and then converted to NSRS2007, before they can finally be converted to NSRS2011. Each of these three conversion processes has its own level of error, and each produces shifts of centimeters in magnitude. The following table indicates the expected magnitude of shifts for the three distinct datum shift calculations:

Datum Shift Calculation	Expected Shift in centimeters
NAD83 --> HARN	40
HARN --> NSRS2007	3
NSRS2007 --> NSRS2011	3

Unlike the NAD83 <--> HARN shift which has now been available for almost 2 decades, the new references systems do include vertical components. The vertical shifts are of the same magnitude as the horizontal shift indicated above. Perhaps this vertical shift is important to some users.

Finally, the new reference system definitions include Alaska which was never included in the HARN series of datum shift files. Quite frankly, I'm unaware of exactly how to convert Alaskan geography from NAD83 to NSRS2007. NSRS models do exist for Puerto Rico and the Virgin Islands, but not for Hawaii or American Samoa for which HARN datum shift files have been defined.

So, the actual significance of this change is questionable. A small portion of the user base is likely to want these features. The implementation of these new geodetic systems no negative effect on the operation of the library other than, perhaps, disk space. The data files used to model the NSRS2007 and NSRS2011 shifts are relatively huge, and each of the three geographic areas covered (48 states, Alaska, and Puerto Rico) requires three distinct data files (longitude shift, latitude shift, and vertical shift). Each of the data files for the 48 states are about 28 megabytes in size. Thus, the size of distribution files could grow to be unreasonably large if full support of the NSRS2007 & NSRS2011 geodetic reference systems is to be included.

Please remember, you can't get to NSRS2011 without first converting to NSRS2007. So, including coverage for 49 states in three dimensions for both of the new reference systems in a distribution will require an additional 456 megabytes of data. It is indeed possible, and perhaps likely, that the data files used in these new reference systems could be de-densifed - a grid cell in all of these files is one minute by one minute; but that will have to be the subject of a new and different RFC.

Proposed Solution

From a technical aspect of view, implementing NSRS2007 and NSRS2011 is rather straight forward. The models for these datum shifts as produced by the NGS is based on a new file format/interpolation combination which the NGS refers to as GEOCON. This format is very similar (almost identical) to previous file formats used, is that used for both horizontal and vertical NSRS datum shifts, and is also now used for the latest geoid height models produced by NGS. Adding a new grid interpolation file format named GEOCON to the repertoire which understands this new file format is relative straight forward; several standard modules are coded and the appropriate entries made in the grid file interpolation table.

Having made the GEOCON grid file format option available, the NSRS2007 and NSRS2011 geodetic transformations can be easily be defined in the Geodetic Transformation Dictionary by referencing the appropriate files.

Implications

This section allows discussion of the repercussions of the change, such as whether there will be any breakage in backwards compatibility, if documentation will need to be updated, etc. Test Plan

How the proposed change will be tested, if applicable. New unit tests should be detailed here??? Funding/Resources?

This section will confirm that the proposed feature has enough support to proceed. This would typically mean that the entity making the changes would put forward the RFC, but a non-developer could act as an RFC author if they are sure they have the funding to cover the change.