Which geotransform to use?

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Which geotransform to use?

Matt Wilkie-2
Hello,

Using ArGIS Desktop v9.2, how does one determine which is the most
appropriate Geographic transformation to be using?

For example, I have some GPS data which I need to line up with Albers
projected data. In the ArcMap transformations dialog there are 13
transformations available, 5 if them with duplicate names, but not
enough information to help choose which one to pick. (see attached image.)

After reading some other threads in the forum (http://forums.esri.com/)
  I discovered the "Geographic_transformations.pdf", but the only
transformation listed for my region specifically, the Yukon Territory,
is for NAD27 to NAD83.

After some more careful reading I find a few NAD_1983_To_WGS_1984 rows
which might be appropriate. However there is not a clean correspondance
between the Arcmap dialog and the pdf. The pdf does not have the
transforms ending with 6,7 & 8 for example. I'm really at a loss as to
how to make an informed choice here. How do I know with confidence which
one should be used?

Thank you for your time.

-matt

Some particulars:

Source:
        Projection:
        Coord system:  GCS_WGS_1984
        Datum:         D_WGS_1984

Destination:
        Projection:    Albers Equal Area
        Coord system:  GCS_North_American_1983
        Datum:         D_North_American_1983

Available transformations:

        NAD_1983_To_WGS_1984_1
        NAD_1983_To_WGS_1984_1
        NAD_1983_To_WGS_1984_2
        NAD_1983_To_WGS_1984_2
        NAD_1983_To_WGS_1984_3
        NAD_1983_To_WGS_1984_3
        NAD_1983_To_WGS_1984_4
        NAD_1983_To_WGS_1984_4
        NAD_1983_To_WGS_1984_5
        NAD_1983_To_WGS_1984_5
        NAD_1983_To_WGS_1984_6
        NAD_1983_To_WGS_1984_7
        NAD_1983_To_WGS_1984_8

--
matt wilkie
--------------------------------------------
Geographic Information,
Information Management and Technology,
Yukon Department of Environment
10 Burns Road * Whitehorse, Yukon * Y1A 4Y9
867-667-8133 Tel * 867-393-7003 Fax
http://environmentyukon.gov.yk.ca/geomatics/
--------------------------------------------

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SUM: Which geotransform to use?

Matt Wilkie-2

Which Geotrans Summary

Dear ESRI-L,

A few weeks ago I embarked on what I thought was going to be a simple single afternoon question: what is the correct geotransform to use when going from Geographic WGS84 to Albers NAD83, in the region of Canada's Yukon Territory?

I'll spare you the ordeal of following the tortuous route I led, and get straight to the results, and then follow with some of the nitty gritty. In the words of Mark Twain, if I had more time I'd write a shorter letter!

SUMMARY

After removing the duplicates, the list of transforms for WGS84 to NAD83 is:

NAD_1983_To_WGS_1984_*
    1 - North & Central America, accurate to centimeters in lower US to ~2m north of 60
    2 - Alaska, Aleutian Islands
    3 - Hawaii
    4 - superceded by #5
    5 - Applies to United States lower 48, accuracy +/- 1m
    6 - Canada, Quebec; (could be restated as "NAD83 to NAD83_CSRS")
    7 - Canada, Saskatchewan; "
    8 - Canada, Alberta; "
For methods 6,7,8 one must contact the regional authority to acquire the necessary .gsb files (see NTv2 heading below):
    #6 - Quebec NAD83-98.gsb
    #7 - Saskatchewan SK83-98.gsb
    #8 - Alberta AB_CSRS.gsb
The best general NAD83 CSRS for all of Canada is: None, use NAD83(Original) instead (either _1 or _5).

For Northern Canada, there is no transformation available between the 2 datum because there were not enough points where both set of coordinates were available to calculate the differences.

The most important thing about selecting a datum transformation is to be consistent, and always use the same transformation when processing your data.

BUGS

DUPLICATE TRANSFORMS: In the ArcMap transformations dialog there are 13 transformations available, 5 if them with duplicate names. This is an acknowledged cosmetic bug, fixed in the forthcoming ArcGIS Desktop Service Pack 3. It doesn't matter which of the dupes is selected, the same transform is applied.

NO ERROR ON MISSING .GSB: the tools don't check whether the NTv2 files are there or not. As long as it sees the geotransform variable is using a value of 1 through 8 it carries on happily regardless. The Defect ID for this bug is CQ00248905 and it listed as a high priority to add a warning message.

Okay, you can stop reading now.

DETAILS

    "The deal is, NAD_1983_To_WGS_1984_1 is a Geocentric Translation, and the transformation parameters are all 0. This treats NAD 1983 and WGS 1984 as if they are identical.

    The actual diference between the WGS 1984 and GRS 1980 spheroids/ellipsoids is 0.4 meters (40 centimeters) of length in the semi-minor axis. Their semi-major axes are the same length. Therefore, at the equator, coordinates on either datum and ellipsoid would be identical, if NAD 1983 were defined at the equator. As the ellipsoids curve toward the poles, because of that 0.4 meter different in the length of the semi-minor axis, the curvature of the ellipsoids separate.

    In southern Florida, for example, the coordinate difference is a few centimeters. At the US/Canadian border, around the Montana/Alberta border for example, the difference is about 0.5 meters. In your area of interest, the Yukon Territory, the data is far enough north where the actual offset between WGS 1984 and NAD 1983 is probably in excess of 1 meter.

    The accuracy of _5, according to EPSG, is ± 1 meter, so within the 48 contiguous states, _1 is actually more accurate than _5, even though _5 is the most accurate transformation that has been calculated to date, since the offset between NAD 1983 and WGS 1984 in the contiguous 48 states is a half-meter or less.

    I've attached a zip file to this email, which contains 3 shapefiles. Each shapefile has two cities in Canada, one in the Yukon Territory, and the other in the Northwest Territory.

    I projected one shapefile from GCS_North_American_1983 to GCS_WGS_1984 using transformation _1, and the other using _5. Comparing the three shapefiles in ArcMap, you can see the differences in the locations of the points.

    The most important thing about selecting a datum transformation is to be consistent, and always use the same transformation when processing your data. Use of either transformation method can be justified."

      Margaret Maher
      ESRI Support Services

NTv2

The Canada National Transformation v2 downloaded separately from ESRI Canada are only for NAD27 to NAD83, not NAD83_CSRS and variants. http://www.esricanada.com/EN_support/1205.asp And yes although the description says 1974(!), this is really is the right stuff. The dates refer to 1974 and 1976(Ontario) defintion or readjustments of NAD 1927.
    NTv2_0.gsb NAD_1927_To_NAD_1983_NTv2_Canada
    May76v20.gsb NAD_1927_DEF_1976_To_NAD_1983_NTv2_Ontario

Regional NAD83_CSRS binary grid shoft files (.gsb)

The regional .gsb files for NAD83 to NAD83_CSRS are handled at the provincial level. Contact list at "Canadian Spatial Reference System, Provincial Contacts" at Natural Resources Canada, Geodesy Branch http://www.geod.nrcan.gc.ca/prov_contact/index_e.php

It sure would be nice at some point to have all of the regional .gsb files available from a central (authoritative) clearing house. Maybe now that geobase.ca is up and running at some future point that could happen, at least for the free distribution ones (Alberta, BC, Quebec, Saskatchwen).

ALBERTA
    Freely available, but must be requested.

    "Some notes about the file (ABCSRSV4.DAC):

    A) We constrained to Alberta's boundary for the points. So, anything outside of AB will have some fluff in it.

    B) Our file is named abcsrsv4.dac --> if you need it with the GSB extension, just rename the file. I know that ESRI have GSB hard-coded into their software. We just called it dac for Direct ACcess.

    C) The file is was created using the same software as for the NTV2 software package, so it's fully compatible in this case."
      -- Geoff Banham, P.Eng., A/Manager Geodetic Control Unit & Land Surveys Unit

BRITISH COLUMBIA MANITOBA
    Does not have a grid shift file for previous versions of NAD83 to NAD83(CSRS) at this time. They are contemplating it, but do not have the necessary coverage in the northern half of the province yet to allow for a proper determination.
      -- David Richards, Manager, Geodesy and Control Surveys Section

NEW BRUNSWICK
    Not available.

    "...The Maritime Provinces have never adopted the original NAD83 (1989) or the Federal Shiba Adjustment done in 1989. We had ATS77 here until 1999 when we officially adopted NAD83 CSSRS as the reference system (datum) for New Brunswick.

    So we developed a ATS77 to NAD83 CSRS grid shift file we use with NBGeocalc or NTv2. There is a non rigorous way to do this with NBGeocalc but I would not recommend it unless it was the one and only way out. One would have to convert NAD83 to NAD27 and then use these to go back to NAD83 CSRS. It will require 2 transformation and the one transformation to NAD27 is based on an old ESTPM version of Ntv2 and accuracy would be lost in the transition.

    If you are looking for mapping grade values, then use the NAD83 CSRS as being the same as NAD83. In most of NB the difference is in the order of 20 to 30 cm so most of mapping data would not be affected by this."

      -- Léo-Guy LeBlanc, Director of Surveys, Service New Brunswick

NOVA SCOTIA
    Free of charge if downloaded by ftp but requires signing and returning a license agreement.

    "We do not have a grid shift file that transforms directly between NAD83 and NAD83(CSRS) - NAD83 was never used as reference frame realization for NS provincial control points. However, Federal first order framework points have NAD83 positions and a grid shift file for ATS77 <> NAD83 transformations using those points was developed. Also there is a grid shift file for ATS77 <> NAD83(CSRS) transformations developed from points of the NSHPN. Thus, I suppose one way to accomplish what you want is, transform NAD83 > ATS77 then, transform the ATS77 > NAD83(CSRS).

    Those transformations are implemented in the web application at:
    http://142.176.62.104/sns_webclient/"
      -- Allen Flemming, Nova Scotia Geomatics Centre,

PRINCE EDWARD ISLAND
    By request, for a fee.

    "You can purchase a copy of PEIGeocalc for $100, which has all of the grid shift files we use, it is our "official" transformation software...but it doesn't have the relatively minor model adjustment from NAD 83 to NAD 83 CSRS, we never needed it so we never had it created."

      -- Serge Bernard, Chief Provincial Surveyor, Department of Transportation and Public Works

QUEBEC
    Free download.

    "We have developed a software called SYREQ to transform coordinates between some datum and some map projections. The software uses GSB grid files. You can download it at http://www.mrnf.gouv.qc.ca/produits-services/syreq/syreq.jsp. Once installed, you'll find the grid files in the C:Program FilesCommon FilesSoftMap Sharedgrid directory. The file you are looking for is NA83SCRS.GSB."
      -- Yves Thériault, M.Sc. Direction de la référence géodésique

SASKATCHEWAN
    Freely available, by request.

    "We offer nad27<-->nad83(nmip91); nad27<-->nad83(csrs98) and nad83(nmip91<-->nad83(csrs98).

    ...nad27<-->nad83(nmip91) is in fact a sub-set of the NTV2 gsb file, in that the Canada-wide NTV2 grid shift file has been trimmed to approximately cover Saskatchewan."
      -- Larry Cook, Head Of Geodesy Surveys Branch

REFERENCES & RESOURCES

Email & Phone:
    ESRI Canada Technical Support: Kimberly D.
    ESRI Support Services: Melita Kennedy, Margaret Maher
    Geodetic Survey Division of NRCAN: Francine Saumure
    University of California: Nicholas Tripcevich, Ph.D.

    Allen Flemming, Nova Scotia Geomatics Centre
    Brad Hlasny, Base Mapping & Geomatic Services Branch, Ministry of Agriculture and Lands
    David Richards, Manager, Geodesy and Control Surveys Section
    Serge Bernard, Chief Provincial Surveyor, Department of Transportation and Public Works
    Geoff Banham, P.Eng., A/Manager Geodetic Control Unit & Land Surveys Unit
    Larry Cook, Head Of Geodesy Surveys Branch
    Léo-Guy LeBlanc, Director of Surveys, Service New Brunswick
    Yves Thériault, M.Sc. Direction de la référence géodésique, Ministère des Ressources naturelles et de la Faune

    Barry Otterson, Government of Saskatchewan
    Linda J. Wright, Surveyor's Office, El Dorado County
    Kilmeny Stephens, Senior GIS Data Analyst, Nelson City Council
> the "Geographic_transformations.pdf" and "PEgt_namewhere.doc"

Usually installed locally at "C:Program FilesArcGISDocumentationgeographic_transformations.pdf" There is an online knowledge base whitepaper of the same which may or not be as up to date: "ArcGIS 9 Projection Documentation" - http://tinyurl.com/2fz5w2

A key to using this document is look at the "Code" column. Transforms are listed multiple times as a single transform can apply to multiple regions. For example NAD_1983_To_WGS_1984_1 applies to all Canada as well as North America. The single transform however will always have the same code regardless of how many places it is applicable.

Also see the similar PEgt_namewhere.doc attached to "HowTo: Select the correct geographic (datum) transformation when projecting between datums", http://tinyurl.com/yustqo

XXXX: code columns don't match betwixt .pdf and .doc is this a problem?


The Grids and Datums column from PE&RS (Clifford J. Mugnier) contains articles written every month with a brief cartographic history of many countries around this world. For people working in countries without published cartographic information (at least published online), these articles can be very useful.
http://www.asprs.org/resources/grids/


ESRI Technical Articles:
    "HowTo: Select the correct geographic (datum) transformation when
    projecting between datums"
    http://tinyurl.com/yustqo
      "...Unfortunately, the list of transformations for ArcGIS 9.2 is not yet ready for publication, but the information is available in the database published by the Oil & Gas Producers Surveying & Positioning Committee. This free database can be downloaded at http://www.epsg.org." -- Margaret Maher

    "FAQ: Where can I get datum transformation grids for Canada?"
    http://tinyurl.com/2h3nzb

    "How do I convert my data from NAD27 to NAD83 using NTv2?"
    http://www.esricanada.com/EN_supportfaq/1262_6303.asp

    "FAQ: Can ArcGIS use NTv2 grid files to convert between datums?"
    http://tinyurl.com/23ztvn

    "FAQ: What National Transformation Version 2 geographic transformation
    grids are supported?" - http://tinyurl.com/ywsqns
    Reference list of .gsb files, their corresponding region,
    transform, and at what ArcGIS version introduced. Example:
    "SK83-98.gsb Saskatchewan NAD 1983 to NAD 1983 CSRS98"

    "HowTo: Determine which NAD_1983_To_WGS_1984 transformation to use"
    http://tinyurl.com/27rary
    Primarily as applies to the continental United States

Geodetic Branch, Natural Resources Canada

CONCLUDING THOUGHTS

This is not an exhaustive summary of all the transforms, though it has been at times exhausting. I only followed the thread far enough to answer my initial problem, namely which transform to use when going from WGS84 to NAD83 in north western Canada. I am by no means an authority on the subject and even now cannot state with absolute confidence I have arrived at an authoritative answer. I only know this is the best I've been able to achieve. Hopefully the base is solid enough for someone else to pick the remaining dangling threads and carry on, and fix the ones I've likely miswired too!

If you only read initial and concluding paragraphs: The most important thing about selecting a datum transformation is to be consistent, and always use the same transformation when processing your data.


Transformally yours,

--
matt wilkie
Geographic Information,
Information Management and Technology,
Yukon Department of Environment
10 Burns Road * Whitehorse, Yukon * Y1A 4Y9
867-667-8133 Tel * 867-393-7003 Fax
http://environmentyukon.gov.yk.ca/geomatics/


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Re: SUM: Which geotransform to use?

Matt Wilkie-2
Hello World,

Thanks to the memory of a co-worker from another department, Julie
Houle, my summary of which geotransform to use have been clearly
elucidated in 3 graphics, which are attached. The maps are taken from
the exercise package of Data Production and Editing Techniques[1].

If I'd had *that* my epic quest would have been answered in minutes
instead of weeks. I'll forward it to ESRI tech support for the next
confused technician who blunders in mumbling incoherently about witchy
dahdum transphomayshuns. I guess this answers the question about whether
ESRI tech support personnel take their own courses!

Oh, just in case you are wondering, those diagrams are worth 4,434 words
(of the summary!), or almost 1500 words each. Expelliarmus!

[1]
http://training.esri.com/gateway/index.cfm?fa=catalog.courseDetail&CourseID=50099067_9.X


--
matt wilkie
--------------------------------------------
Geographic Information,
Information Management and Technology,
Yukon Department of Environment
10 Burns Road * Whitehorse, Yukon * Y1A 4Y9
867-667-8133 Tel * 867-393-7003 Fax
http://environmentyukon.gov.yk.ca/geomatics/
--------------------------------------------


Nad83ToWGS84.pdf (265K) Download Attachment
Nad27ToNad83.pdf (707K) Download Attachment
Nad27ToWGS84.pdf (695K) Download Attachment