Background: Downloading GPS data stored in WGS84/NAD83 datum and displaying them with GIS data in NAD27 datum can result in a 100's of meters in error! Here is a guide through the Datum tarpit. What follows is an Alaska approach. Parks in Conternimus US (lower 48) can utilize substitute CONUS instead of NADCON ALASKA under step-by-steps. This version includes latest DNRGarmin instructions and fixed some errors.
First some Definitions:
Datums: See http://gpsinformation.net/main/maps.txt
Datum Transformation Engines: Software designed to transform coordinates from one datum to another. Tools include Corpscon, Nadcon, Blue Marble's Transformer, Trimble Pathfinder Office that live outside of GIS, or extensions that can be called from within GIS. The latter may include, ArcGIS Toolbox Transform, ArcView's projection wizard or the NADCON extension (http://www.pierssen.com/arcview/upload/old_30/nadcon.htm)
Datum Transformation Errors: These errors range from 3 to 1000 meters. This is a result of improperly using the wrong datum (guessing data is NAD27 when its not) or using an less robust Datum Transformation Engine like the Molodensky datum transform. Knowing how to transform the datum is as important as knowing the datum.
NAD83 "Leap of Faith": For the sake of discussion, whenever you hear WGS84/NAD83, you can automatically assume it is NAD83. In this document we will refer to either WGS84/NAD83 or NAD83 as WGS84/NAD83. The differences between these two datums for North America is not discernible with mapping/GIS grade or consumer grade GPS equipment. Interchanging these datums in your head is essential for using the tools available on the market today. To read more about this see a discussion thread on ArcGIS: http://forums.esri.com/thread.asp?c=93&f=984&t=60652#204248
Above all, teach “Know thy Datum”. To make it though this tarpit, knowing the datum is the most important step and can save hours of troubleshooting. If you communicate a coordinate on paper, radio or spreadsheet please write the DATUM down somewhere. If you use a Software (like DNRGarmin, ArcView, ArcGIS, Corpscon) to download the GPS data the rest of this document will help you in deciphering what is going on during the conversion.
Read on for more technical datum transforms by gps reciever and software types
It is assumed that you are bringing GPS data from the field into a GIS based on the NAD27 datum. This means you may be tasked to transform raw GPS data in the form of WGS84/NAD83 to NAD27 for display with other NAD27 Datum data. Depending on the GPS Tool and Software, here are our recommendations:
Data received in a table:
You may require tabular transform on coordinates before bringing into a GIS. The basic steps for bringing in tabular coordinate data is converting to UTM or Decimal Degree format and using “Add Event Theme” in ArcView or “Add XY” in ArcGIS. If your GIS lives in NAD27, then converting the WGS84/NAD83 data may make sense using a program outside of GIS. Corpscon is one of our favorites.
Corpscon for Windows http://crunch.tec.army.mil/software/corpscon/corpscon.html. See the AKSO cheatsheet called “CorpsconUse.doc” for instructions
You might have data in a Degree Minute Second format (e.g. 148 23 44.33). To convert this coordinate system format to decimal degree for importing into GIS remember these steps:
http://www.dnr.state.mn.us/mis/gis/tools/arcview/extensions/DNRGarmin/DNRGarmin.html . As of Fall 03’ Please use the DNRGarmin42Beta Version (10/07/03) for the latest in handling projections. Previous installs of DNRGarmin must be removed before new installations. Go to the DNRGarmin Installation Help Page for more info.
NMEA protocol, then switching the datum on the Garmin display will change the datum on output so transforming data may not be necessary. Read more http://gpsinformation.net/main/datumout.htm . You can also use DNRGarmin for realtime tracking.
Aircraft / Animal Borne GPS:
Most panel-mounted GPS in aircraft by default use WGS84/NAD83 Datum. You must assume all data written down or transferred by cable is WGS84/NAD83. Animal borne GPS collar data (ATS, Telonics etc) will also be in raw WGS84/NAD83 datum. Even though this type of GPS data can be 100's of meters off depending on habitat types http://www.atstrack.com/Support/Literature/GPS_Accuracy_Study.pdf
you still want to minimize what error you can control. Transform text data using Corpscon or by using a transform inside ArcView or ArcGIS or risk doubling error in some cases.
1) All data downloaded to Pathfinder office (*.ssf) from a Trimble GPS is in WGS84/NAD83, Decimal degrees. Data is merely transformed “on the fly” in the Map window. Exports in Pathfinder Office allow users total control for switching datums.
2) For Alaskan ArcView users you must use Latitude Longitude as the System and NADCON (Alaska) as the Datum in order to display your data with the NPS standard. DO NOT USE NAD 1927 (Alaska), which utilizes the Molodensky transform. When selecting other coordinate systems, always ensure you are using the NADCON Datum version. Conterminous US users must use NADCON CONUS. DO NOT USE NAD 1927 (CONUS)
ArcView Projection Wizard (ver 3.2 or 3.3):. ESRI could not have made a more cumbersome transformation engine. You must make the NAD83 "Leap of Faith" to get the best results. The Alaska Support Office have cheatsheets available by request. This process assumes you have Decimal degree, WGS84/NAD83 data. As before, these instructions are written for Alaskan NPS users to properly project their data for use in the NPS standard:
1) Under STEP 2, select Geographic as Coordinate System Type and "GCS_North_American_1983 as Name. Units are Degree.
2) Press the Show Advanced Button and IF IN ALASKA select "NAD_1983_TO_NAD_1927_Alaska" as the Geographic Transformation. The transformation used will be NADCON 
IF NOT IN ALASKA select "NAD_1983_TO_NAD_1927_NADCON" as the Geographic Transformation. The transformation used will be NADCON 
3) Under STEP 3, select Geographic as Coordinate System Type and "GCS_North_American_1927 as Name. Units are Degree.
ArcGIS Toolbox (Ver 8.2): Yet another ESRI approach that frustrates what you see in ArcView is not what you do in Arc. This process assumes you have Decimal degree, WGS84/NAD83 data. This process is written for producing unprojected (Decimal Degree vector data).
1) Under Projections / Define Projection Wizard (shapefiles), Select from the predefined coordinate system, Geographic Coordinate Systems / North America / North American Datum 1983.prj. This is when you need to make another NAD83 "leap of faith".
2) After applying those changes, now enter the Project Wizard (shapefiles)
3) You should now see that the shapefile is "GCS_North_American_1983. Now time to select the output. After specifying a name, Select from the predefined coordinate system, Geographic Coordinate Systems /North America / North American Datum 1927.prj.
4) After applying these changes, you will need to Select the appropriate Datum transform for going from WGS84/NAD83 to NAD27. Press the Set Transformation… button…
5) Your windows should now say From "GCS_North America_1983 and To "GCS_North_America_1927". In the “Using” pulldown box, select "NAD_1927_To_NAD_1983_Alaska". Is this getting crazy or what!. Regardless if you are going from 83 TO 27, you use the 27 TO 83 method - sheeeezzzzzzzzzzz.
ArcPad (Ver 6.X): In order to tell ArcPad what projection your data is in, you must assign a projection file for everything. Wash your data through either ArcView or ArcGIS and be sure to assign a correct prj file. ArcPad on the fly will then assume you did the right projection and create output shapes in the same projection as your other projected data.
NOTE: Keeping a set of projection files (*.prj's) around can make you life a lot easier. The two critical ones if you are working with Decimal Degree data are the NAD83 and NAD27 prj files produced from the ArcView or ArcGIS Toolbox Wizards. Whenever you get Raw GPS data in WGS84/NAD83, copy the appropriate one alongside the shape a NAD83 prj file and you can skip the project-defining step.
Keep these basic steps in mind and you should be able to keep GPS data in the best shape possible. Now if we can just get Alaska to switch to WGS84/NAD83. :> ....Good Luck