Common Questions about GPS
Source: Based on Kerski, J. 2004. Questions and Answers Regarding GPS,
http://rockyweb.cr.usgs.gov/outreach/gps/gps_questions_and_answers.html, accessed 10
July 2005.
1. Getting GPS to work properly
a. Choose either Universal Transverse Mercator System (UTM) or
latitude/longitude.
b. Choose the correct datum. A datum describes the size and shape of the
earth in reference to the coordinate system (lat/long, UTM, state plane)
used to map the earth. Note that while the WGS 84 datum is commonly a
default on many GPS units, USGS topographic maps are often NAD 27
unless indicated otherwise.
c. Select true north, magnetic north, or grid north.
Note that the vertical dimension, because of varying datums and satellite signal blockage
will always be less accurate than the horizontal dimension. Don’t use recreational units
for any decision involving elevation or altitude. Vertical elevations are often off by over
400 ft.
2. What is UTM?
a. Universal Transverse Mercator, which is based on the Mercator map
projection.
b. Beginning at 180 degrees longitude, there are 60 UTM zones increasing in
6 degree increments.
c. Any UTM zone is divided into north and south zones.
d. The state of Michigan has two UTM zones, 16 N and 17 N.
e. On topographic maps, UTM grid marks are indicated with blue tic marks.
f. To eliminate the necessity for using negative numbers to describe a
location, the east-west origin is placed 500,000 meters west of the central
meridian. This is referred to as the zone’s ‘false origin’. The zone doesn't
extend all the way to the false origin.
g. Discover the your UTM zone at Alan Morton’s (2005) UTM Grid Zones
of the World http://www.dmap.co.uk/utmworld.htm (accessed 10 July
2005).
3. What are examples of how a position is reported completely and correctly?
a. Latitude/longitude – NAD 27 CONUS, 44 degrees, 16 minutes, 39
seconds north
b. UTM – WGS 84, zone 16 N, 330456 E, 4480111 N
c. UTM coordinates are read to the right and then up.
4. How much does a degree, minute, and second of latitude and longitude vary on
the globe.
a. Latitude does not vary much. At 38 degrees north one degree of latitude
equals about 69 miles, one minute, 6068 ft. and one second, 101 ft.
b. Longitude is highly variable depending on which line of longitude
(parallel) is used. Values range from 0 at the north pole. At 38 degrees
north one degree of longitude equals about 55 miles, one minute. 0.9
miles, and one second, 80 ft.
5. What most often explains discrepancies between map and GPS positions?
Differences in datums.
6. What is the accuracy of GPS units?
a. Low end units (less than $500) 30-60 ft
b. Mid range units ($4-5 K, with differential correction capability) 3 to 30 ft.
c. High end units($10-12 K, differential correction “on the fly”). Less than 3
ft
d. Survey grade instruments > $60 K cm to mm accuracy.
7. Should I set my GPS unit to magnetic or true north?
a. Select which “North” your GPS receiver will use as the zero degree
reference. The default in your receiver is probably true north = 0 degrees.
You can also select magnetic north = 0 degrees or grid north (north-south
lines of the UTM grid) = 0 degrees. Many users select magnetic north as
zero degrees to they can follow a compass bearing without converting the
azimuth to true north. Many GPS units automatically calculate the local
magnetic variation (declination) for the fix time and date
b.
8. What are the different coordinate systems in which GPS points are most often
reported?
Coordinate System
Abbreviation
Example
Universal
UTM
Transverse Mercator
16 N, 330456 E,
4480111 N
Decimal Degrees
58.65375° N,
-158.65375° E (note
that a negative sign
is used for west
longitude)
d
Can directly be
imported into most
GIS programs?
yes
yes
Geographic
Coordinates
GC
27° 30' 22" N, 99°
30' 26" W
Decimal Minutes
DM
58 ° 39.645′ N,
-128° 59.284 W
Degrees Minutes
Decimal Seconds
DMS
58° 39′ 13.5″ S,
5° 05′ 99.5″
State Plane
SPC or SPCC
Easting 200487.391
m, Northing
5204194.769
meters; Zone: 4205.
Michigan GeoRef
No, should be
converted to UTM
or d
No, should be
converted to UTM
or d
No, should be
converted to UTM
or d
yes
yes with ARCGIS,
yes in ArcView with
free download
9. How can I convert DM and DMS to d?
a. DM to d example: 58° 39.225′
d = 58° + 39.225°/60 = 58° + 0.65375° = 58.65375°
b. DMS to d example: D = 58° 39′13.5″
formula d = D + M/60 + s/3600
d = 58° + 39°/60 + 13.5°/3600 = 58° + 0.65° + 0.00375° = 58.65375°
Ans: 58.65375°
10. What web conversion programs exist for converting data between coordinate
systems? State plane to http://www.ngs.noaa.gov/cgi-bin/spc_getpc.prl
http://www.ngs.noaa.gov/cgi-bin/spc_zones.prl
11. How do I know what zone I’m in if using UTM and State Plane Coordinate
Zones? Michigan has three zones:
a. UTM: Morton, A. 2005, UTM zones of the world,
http://www.dmap.co.uk/utmworld.htm, accessed 12 July 2005.
b. State Plane: Mentor, 1985, State Plane Codes,
http://www.mentorsoftwareinc.com/RESOURCE/stplane.htm accessed 12
July 2005.