Here are Excerpts from a Forest Service document regarding gps use. The excerpts are taken from a large document but contain key information that can transfer
to the recreational use of a gps unit. This is getting Nerdy
For several years, GPS data have been collected by the Forest Service. In the early years of GPS, data were frequently collected in NAD27 so that they would agree with NAD27 base maps (e.g., topographic and other maps that were produced in NAD27) without any additional effort. In 1992, there was a national mandate to switch geographic datums to the NAD83 datum. For various reasons, this has not been done consistently across the agency. Recently, the Geospatial Advisory Committee has drafted a memo reaffirming this mandate, and has created a timeline for the final mandatory conversion. Conversion of legacy GIS vector data stored in NAD27 or other datums to the NAD83 datum will entail using standard GIS transformation methods
For those who use NAD27… set the datum to NADCON in GPS applications using NAD27 coordinates
when possible. Not using this method for NAD27 data can result in several meters of error. NOTE:
There are WGS84/NAD27 transformation issues within ArcPad and ArcMap (see Appendix J).
This document does not discuss all the complexities involving projections and datums. Instead, it
simply outlines what projection and datum need to be used depending on the data situation. The
following table mainly pertains to Forest Service’s use of data in the NAD83 datum in Region 2.
USGS Topo Geographic (Lat/Long) NAD83 quad tics can usually be found offset from the NAD27 tics.
Currently, USGS topos do not have NAD83 UTM tics.
GIS data for Region 2, GIS data resides in UTM, NAD83 Conus, Zone 13 N, meters
GPS receivers when collecting data, most GPS receivers store positions in Geographic (Lat/Long) WGS84
Waypoints when creating waypoints, one must keep in mind what projection/datum the source coordinates are
in, especially when entering the coordinates by hand. Geographic WGS84/NAD83 coordinates are
different from Geographic NAD27 coordinates.
GPS Analyst/ArcPad The correct datum transformation will need to be used based on the situation to correctly move
between WGS84 and NAD83 to remove the 1 meter difference between the datums.
D. Waypoints
Many times, it is useful to upload GIS data to a GPS unit for use in the field, whether for updating
information or for simple navigation. It is important to understand a little about projections (see
Section I, part B) when preparing data for navigation back out in the field.
When entering waypoints manually, set the GPS/GIS projection to match the projection of the source
coordinates of the point to enter. For example, if entering a Lat/Long NAD27 coordinate in Pathfinder
Office that originated from GIS or a Topo, you would set the Pathfinder coordinate system to Lat/Long
NADCON. If entering a Lat/Long NAD83 coordinate that originated from the GPS unit that was set to
Lat/Long NAD83 Conus, you would set the Pathfinder coordinate system to Lat/Long NAD83 Conus.
You can create and upload Waypoint files to a Trimble GPS unit using Pathfinder Office.
Garmin GPS12 UTM points appear to be rounded to whole numbers and tenths for Lat/Long. A point in
UTM NAD83 does not necessarily match the same UTM NAD83 point entered on a Trimble. There is at
least couple meters difference. Also, be aware of the difference between Geographic WGS84, NAD83
and NAD27 coordinate values.
I need to write up and example on how the above can benefit a hunting planning a route up a mountain using a topo, software on a computer and then loading the route on to you gps. F. Fire (Incident) Application
More and more, the firefighting community is using GPS as a tool for mapping and navigation. The wide
variety of basic GPS receivers, relatively low expense and interface with GIS applications has increased
the need for GIS specialists (and average users) to become more familiar with this type of data
management and manipulation. Garmin receivers are probably the most common units found on an
incident.
Unlike GIS specialists who normally work in projected coordinates (UTM, State Plane), the fire
community typically operates in Geographic (Latitude/Longitude) coordinate space. For standard
communication purposes, the fire community (at least in Region 2) is supposed to provide coordinates in:
Latitude/Longitude
Degrees, Minutes, Decimal Seconds (DD° MM’ SS.ss”)
However, for communication with aircraft, the format is typically:
Latitude/Longitude
Degrees, Decimal Minutes (DD° MM.mm”)
These standards do not stipulate a datum. A GIS specialist is often asked to plot these coordinates on a
map. For this reason the GIS user should always ask the person providing the coordinates:
“What is the coordinate system? What is the datum?”
This only applies to verbal or hand-written coordinates. If data was physically collected within the GPS
unit, it usually exists as Geographic WGS84 data until it is downloaded into GIS.
b. Coordinate System for GPS Unit
The user should select a coordinate system they are most comfortable with for display
purposes. If there are several units assigned to a particular project/resource unit, then all should
be set to the same coordinate system. This should be decided by the project or resource unit
leader. It does not matter what coordinate system is set on the GPS as it always stores data in
Geographic (Lat/Long) WGS84. The user will convert the data into a desired projection using
the export function in Pathfinder Office.
NOTE: The coordinate system can make a difference when entering waypoints or referencing
a map. The user needs to keep in mind the coordinate’s source when manually entering a
point.
Coordinate System: LAT/LONG
Datum: NAD83 Conus
Altitude Reference: MSL
Altitude Units: FEET
C. GPS Datum Transformation Setup
GPS data can shift up to several meters from the correct location when not using the correct datum
transformation from WGS84 into NAD83 or NAD27.
I left out a bunch of Nerdy stuff, but a little knowledge of the above maybe helpful.
A while back I was explaining gps use to a hunting partner. He is a very bright boy whose come back was that he is a country hick. But a few days ago I recieved an e-mail asking about gps units and cellphones. This was kicked off because of the S&R operation for the Kim family in Southern Oregon. That event because of the related publicity has drawn a lot of attention to gps and cellphones. Just remember that if you buy into a gps/cellphone navigation plan you need to be in your providers service area, and you need to hit a cell tower. That is rather difficult in the Western Mountains. And my brother-in-law says they have the same problem in the Southern Rockies. Here is a link to one providers package.
http://estore.vzwshop.com/vznavigator/?cm_mmc=Verizon+Wireless-_-Handset+Post+Upgrade%5BHQ0001%5D-_-Nokia+6315i+%2D+West-_-Hero+Bullet+3+Copy&cm_em=b_sopp@yahoo.com