QUESTIONS FREQUENTLY ASKED
OF THE AUTHOR
What
program do you use to analyze the GPS data?
The author does not use a
program specifically designed for doing such.
He is a mathematician and uses numerical/statistical software with which
he is familiar, rather than specialized software. Most of the analysis and
graphs were done with statistical/numerical software PSI-Plot, from Poly
Software International, Inc. in Pearl River, NY, but I only recommend this if
you know what you need to calculate.
The autocorrelations were calculated using ProStat, which is also
available from Poly Software International, Inc. Most of the data logging was done using programs written using
PowerBasic. The web pages were written using Microsoft Word with cut/paste from
PSI-Plot and saving the Microsoft Word file in html format.
PSI-Plot - http://www.polysoftware.com
PowerBasic - http://www.powerbasic.com
Why do you
not test the GPS model ...?
The author's
time and resources are both limited to those configurations that interest him.
The tests
of various GPS receivers was not a fair comparison because ...
Why not do
a fair test?
Different GPS receivers are
different. There is really no such thing
as a fair comparison. The results
presented should be understood to be simply measurements of various
configurations and not a contest to determine which receiver is best. As accuracy is the main subject of these
pages and other features are often of importance to users, buyers should also
consider those features, although they are not the focus of these web pages.
What about
accuracy when one converts between datums?
GPS uses WGS84. The internal datum conversions in GPS
receivers usually use the "Molodensky transform." That is only an approximation (and there are
better ones). When desiring U.S. NAD27
coordinates from the GPS WGS84 or NAD83 coordinates, using Molodensky (which
includes the programs MADTRAN, DT&CC and GEOTRANS) may yield undesirable
errors. In that case, recommended
software includes CORPSCON, NADCON, or perhaps the trial version of
Tralaine. All three use large databases
that give good accuracy for the United States.
In the rest of the world, Molodensky or similar approximations are often
necessary. The program HTDP will
convert from WGS84 to/from NAD83 and also includes various ITRF datums.
CORPSCON - http://crunch.tec.army.mil/software/corpscon/corpscon.html
HTDP - http://www.ngs.noaa.gov/PC_PROD/pc_prod.shtml
NADCON - http://www.ngs.noaa.gov/PC_PROD/pc_prod.shtml
What are
the equations for finding distance and bearing between positions?
I recommend two pieces of
software: DIRECT/INVERSE (with FORTRAN source code available) from NGS and
Mentor Software's "freebie" FORWARD/INVERSE.
INV/FWD3D - http://www.ngs.noaa.gov/PC_PROD/pc_prod.shtml
FWD/INV - http://www.mentorsoftwareinc.com
These programs can also be
used to answer the frequent question:
"How many meters per second of latitude or longitude?"
What
program will find the geoid height?
The Mentor
"freebie" GeoidHeight will do this and the new program NGS Geoid99
gives the up-to-date accuracy in doing so.
These programs are good only for the United States.
GeodHeight - http://www.mentorsoftwareinc.com
Geoid99 - http://www.ngs.noaa.gov/PC_PROD/pc_prod.shtml
Is there
software for converting between geographic and magnetic directions?
Ones needs the magnetic
declination, which is the difference between the two. This can often be found
on aeronautical maps. For locations in the United States, the program Geomagix
will also do this.
Geomagix v1.0 (freeware) - http://geomag.usgs.gov/frames/mag_mod.htm
Geomagix v2.0 (shareware) - http://www.interpex.com
Is there
software to convert between degrees/minutes/seconds and decimal degrees?
Since 60 seconds=1 minute
and 60 minutes=1 degree, this can easily be done on a calculator.
If you need software, Mentor
Software's "freebie" DegMinSec will do the job.
DegMinSec - http://www.mentorsoftwareinc.com
What other websites are of interest to those
seeking GPS information?.
Rather than attempt to list
them, the reader is referred to two excellent sites that have additional
information and links to many other relevant sites:
http://www.cnde.iastate.edu/gps.html
( Return to http://www.erols.com/dlwilson/gps.htm
)