**Use of ArcPad in a NAD27 World**

December 2003

I have fielded several calls from folks using ArcPad 6.X on handheld
devices and slaved to GPS units (consumer or GIS grade) while collecting
data on top of their GIS data based on NAD27 Datum. Not to throw some
hot water on this, but thats a bad idea.

This is not recommended since ArcPad can only use a lesser quality datum
transform (Bursa-Wolf) for all its on-the-fly projections resulting in
inaccuracies that may exceed the error in the GPS solution. The result
is base data probably projected using NADCON (the right way) and adding
to it data transformed using equation based(the wrong way). I have
included a more technical thread below and a hyperlink to an ongoing
ESRI ArcPad forum on NAD27 and ArcPad. Take away message is this:

\* Don\'t generate NAD27 shapes with ArcPad

\* Make the Switch to NAD83/WGS84 in your Parks GIS before launching
down mobile GIS

\* Lean on ESRI to use NADCON grid based datum transform - excuse of
\"its too memory intensive\" just does not cut it anymore on modern
handhelds.

\* If you have to use ArcPad on devices loaded with NAD27 based GIS, use
ArcPad\'s TrackLog feature that irregardless of the ArcPad maps
projected display always produces NAD83/WGS84 Decimal Degree data. You
can convert these to NAD27 back in the office using NADCON strength
projection. Brian Sorbel has a good Cheatsheet on this approach.
http://www.nps.gov/gis/gps/aksogps/GenGPSToolKit03/Cheatsheets/Arcpad_how_to.pdf

The Problem:

ArcPad supports only transformation parameters for going to / from
WGS84. ArcPad does no conduct grid transform (NADCON) and must apply an
equation based transform (like the Bursa Wolfe). You can modify the
apDatums.dbf located in your ArcPad installation folder only if they
meet the criteria above.

The Confirmation:

Here is a very active thread at the ESRI site for ArcPad - it has to
deal with using ArcPad with NAD27 data. Click on this link and see how
much work this is bringing to the GIS community, including quotes like
this:

\"\...sic Here is what I did. I used ArcToolbox for Arc8.1.2 and
reprojected my ArcPad NAD27 shapefile back into WGS84 using the New
Geographic Transformation option (when it asked for the transformation)
and hand entered the -4, 135, 181 numbers. Once I had a WGS84 shapefile,
I reprojected it into the correct NAD27 datum for my area. The shapefile
lined up where I expected it to. \"

From http://forums.esri.com/Thread.asp?c=34&f=1172&t=101030#303490

Here is a quote by Melita Kennedy (ESRI\'s Queen of Projections) -
August 5, 2003

**This paragraph from Melita is essentially saying. \"Don\'t generate
NAD27 shapes with ArcPad !!!!!!!!!!!**

> **ArcPad only supports three and seven parameter datum transformation
> methods. Unless you can calculate a local seven parameter
> transformation for your data, none of the predefined transformations
> are really that good. The best transformation is the NADCON method
> which uses two files of shift values to model the differences between
> NAD27 and NAD83. The files are generally too large to support on
> hand-helds.You really should be using the NADCON method to convert the
> data. In general, NAD83 and WGS84 can be treated as if they are the
> same. \-\--Melita **

About the author:

\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*

Joel Cusick

National Park Service

GIS Specialist/ GIS Team/ Alaska Support Office

240 West. 5th Avenue

Anchorage, Alaska 99501

\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*