GPS Myths

Alaska Support Office GIS Team

REVISION LOG

  ---------- -------------------------------- ------------- ---------------
  Revision   Description of Change            Author        Effective Date

  9/14/04    Initial creation                 j\. cusick    9/14/04

  2/2/04     Edits per Mike Londe comments    j\. cusick    2/4/04

  7/5/09     Datum, one more time             j\. cusick    7/5/05
  ---------- -------------------------------- ------------- ---------------

This document is designed to dispel GPS collection myths and give sound
advice for digitizing with GPS receivers intended for GIS.

**Myth: There is no rush to convert my GIS to NAD83 anymore since ArcGIS
can Project-on-the-Fly. I'll just keep exporting GPS data to NAD27 and
let ArcGIS figure it out?**

*Reality:* GPS inherently stores data in WGS84(G1150) Datum and by
exporting an earth-centered coordinates into a spheroid model of the
earth developed in 1866 (before we had satellites), which NAD27 is based
on, you are corrupting an otherwise incredibly accurate and precise
digitizing tool. GPS has so fundamentally altered the way data is
collected, any GIS shop still based in NAD27 datum is causing positional
degradation, delaying the inevitable and not allowing GPS quality to
come thru. By relying on ArcGIS's Datum Transformation on the fly, you
can mix datums in the same data frame (not a good idea) and users can
unknowingly export data BACK to NAD27. Switching to NAD83 today will
take some time -- but the payoffs are incredible.

**Myth: My recreational grade GPS always gets data when my expensive
receiver does not. Is it worth to keep this expensive rock around any
longer?**

*Reality:* GPS units vary in the ability to see GPS signals and allow a
position to be collected. Usually, for GIS data collection purposes (at
least in the NPS), the horizontal position needs to be within a couple
of meters from the true location. More expensive mapping/resource grade
receivers can be set to automatically filter out poorer quality
positions. Recreational grade receivers will allow a position to be
collected at any level of horizontal positional accuracy and do not
store any estimates of error. Some software (like ESRI's ArcPad) does
allow a user to set a PDOP or HDOP mask, but not all.

**Myth: The GPS unit is the most important item to purchase. Software is
secondary.**

*Reality:* Not anymore. With Windows CE-based dataloggers the choices of
mix/matching the GPS receiver, the datalogger, CE-based GPS software and
PC software must be considered before deciding on a "GPS unit". A GPS
unit may be cheap, but after you add the software, training and cabling
to get a "system" to work, you may equal the amount of money on a more
expensive GPS system that pre-plans, structures the attribute
collection, collects and filters the GPS data and post-processes for the
GIS. Don't get caught up in going cheap, since the real work begins
AFTER you have collected the GPS data. Some GPS solutions leave you with
limited means to pass into the corporate GIS, high quality data and
accuracy statements for documentation.

**Myth: Now that I purchased a receiver with WAAS, I don't need to
augment GPS positions with realtime DGPS equipment or post-processing.**

*Reality:* Relying on WAAS for 100% of your DGPS needs is not realistic.
WAAS is designed by the FAA to guide aircraft down to 200' above the
surface of the earth. WAAS differential will not work effectively under
tree canopy and when the southern sky is occluded. Relying on WAAS for
your DGPS needs will introduce data into your corporate datasets that
may be more accurate sometimes and not so accurate other times. Low end
receivers do not track WAAS consistently, often requiring 20 minutes to
lock. How will you know? A better solution is investing in real-time
differential systems or those systems that utilize post-processing
differential. Using equipment that post-processes ensures 100%
differential processing for your GIS needs.

**Myth: All I need is the GPS receiver and no external antenna.**

*Reality:* The key to receiving high quality GPS data is a clear view of
the sky and an un-interrupted strong signal from at least 4 satellites
in a good geometric pattern overhead at any one time. Since many mapping
missions may be in hilly or mountainous regions, under tree canopy or
from within a moving vehicle, holding a GPS unit in your hands can block
crucial access to satellites. An external GPS antenna placed above a
mapper's head will maximize the greatest number of satellites to be used
in a GPS solution, and often times increase your efficiency in poor GPS
environments. An external antenna also allows you to free up your hands
to write things down or place the GPS receiver in a storage location
(like a pocket) during long stretches of monotonous terrain*.* We
encourage buyers to research the accuracy differences between their GPS
receiver's internal and external antenna and buy the best one available.

**Myth: With GPS accuracy getting better and better, there is no reason
to average a point location. Just collect an instantaneous position and
move on.**

*Reality:* If you are not investing in any other error correction method
discussed above (real-time or post-process differential), averaging is
your best chance in generating a more accurate position due to simple
math. GPS errors are still present and fluctuate; averaging these
multiple positions provides a more accurate point.

*Purchase GPS units that Average*: The following Garmin GPS receivers
allow averaging (Garmin III+, 12XL, Map76, 60C). The eTrex series cannot
average. All Trimble receivers average positions by default.

*Log enough positions*. Be sure to log at least 180 positions (For
Garmin receivers, that's 3 minutes at 1 position per second.)

*Stand still, but wiggle!* This may sound silly, but when averaging a
position, you must be not moving. However, in poor GPS environments,
like a dense forest, a small movement (a wiggle) of the antenna may
provide you a position that a few seconds ago was not possible. These
subtle antenna movements can provide the crucial signal that allows a
position when other GPS quality filters (like PDOP/HDOP, SNR, Elevation
Mask) are set appropriately for the situation.

**Myth: I simply don't trust storing my GPS locations on the GPS. I
write all my locations down and transfer them into GIS when I get back
to the office. I think this is the best route:**

*Reality:* This is a bad idea. Not electronically storing a GPS position
places one in the situation where datum's, coordinate formats and
accuracy can all be dropped or confused when entering into a GIS. User
error now vastly exceeds GPS error especially since electronic storage
of GPS data in receivers is tightly controlled in certain coordinate
formats, significant digits and WGS84 datum allowing for the best
position possible. Once the data is stored internally in the GPS, the
appropriate software used back in the office will ensure that the data
is transferred in the highest quality possible. Contact your park's GIS
coordinator to find out what software should be used to download the
GPS. If you must write down a coordinate, then you must write down the
datum chosen on the GPS screen display, units and estimated error during
collection.

**Myth: I don't need to worry about coordinate systems and datums when
using ArcPad.**

*Reality:* Three questions must be ascertained about coordinate systems
and datums before you invest in an ArcPad solution for your GIS needs.
Not asking these questions first will cause considerable heartache, pain
and errors when merging ArcPad data back into your GIS.

*Question 1: What is the datum of your GIS Layers?* You should have PRJ
(Projection) file assigned for all your GIS layers BEFORE using ArcPAD
and they must be all in the same projection. This is because ArcPad
"listens" to the PRJ file to assign how to transform GPS input
on-the-fly into a shapefile. If you add a layer with no projection (.prj
file) to a map with a geographic projection, ArcPad checks if the data
appears to be lat-long (geographic) and will assign those data a datum
from the apDatums.dbf file. If multiple datums exist with the same name
(there are 21 variants of NAD27), then you must use the Select Default
Datum tool in the Layers dialog box to select the datum you want.
Incorrectly defining the layer's datum in the field can cause transform
errors in relation to the GIS layers back in the office. Be very careful
and ensure you use ArcGIS to change the projection of your data BEFORE
adding it to your ArcPad map.

*Question 2: What is the datum of your incoming GPS positions from the
GPS receiver?* ArcPad has no control over incoming GPS data, but relies
on GPS manufacturer protocols to send data in certain formats and
datums. Garmin receivers set to NMEA protocol are dependent on the
screen display of the datum. Altering the datum on the Garmin can alter
the GPS input so that ArcPad receives data in the wrongly assumed datum!
Always set any Garmin in NMEA protocol to WGS84 Datum and never change!.
You do not need to worry about TSIP protocol receivers (Trimble), or
PLGR's since their GPS input is always WGS84.

*Question 3: Are your GIS layers in NAD27?* If so, you should not be
using ArcPad for high resolution, on-ground mapping. Since ArcPad cannot
use NADCON, a high quality Datum transform engine, considerable error
may result in your GPS positions (1 -- 15 meters). In order to fully
utilize GPS technology for your high resolution GIS mapping needs, you
should transform all your Park data to NAD83 as soon as possible. Not
only will this remove datum transform errors inherent in the older
spheroid/datum based coordinate systems, but ensure that your GIS is in
the best position possible to receive data from the most incredible
digitizing tool we have -- GLOBAL POSTIONING SYSTEMS.