Finding Your Latitude and Longitude
How you go about finding your latitude and longitude depends to some extent
on whether you want to know your 4-digit grid square or your 6-digit grid
square. Finding a 6-digit grid square requires considerably more precision in
latitude and longitude. The 4-digit grid squares covers a pretty big area, while the 6-digit grid square
is fairly small. The actual size of a grid square depends
on the particular location, as the 1° of longitude represents
different distances at different latitudes; longitude lines are closer
together at the Earth's poles than at the equator.
The first step, then, is finding your latitude and longitude as accurately
and precisely as
you can. There are several common methods for doing so:
- Lookup your
Lat/Long online at the
US Geological Survey GNIS website. Enter your town in the "Feature Name" box and select the
appropriate state before clicking on the "Send Query" button. It will return a list of features that match the name
you entered, along with the lat/long for each one.
- Use a topographic (or topo) map. These maps contain both elevation data, by
means of countour lines, and location via coordinate systems. Topo maps of
US areas are available from the US Geological Survey
(USGS). The most useful
series of maps for finding latitude and longitude are the 7.5-minute maps,
each of which covers an area of 7.5 minutes of longitude by 7.5 minutes
of latitude. Most of these are at a scale of 1:24,000 (1 inch = 24,000
inches, or 2,000 feet).
These maps usually include two coordinate systems to define location. One
system uses latitude and longitude-- this is what we want. The other system
is the universal transverse mercator (UTM) system that consists of
squares 1 kilometer on a side. For finding your grid square, use only the
latitude and longitude system.
An online topo map is available at
the U.S. Census Bureau website.
To obtain the 7.5-minute map that covers your location, you need to
determine the reference code, map name and state. You'll find this
information in the map index for your area. Contact USGS for information
about pricing and
how to order topo maps.
Many outdoor, sporting-goods and camping stores also sell USGS 7.5-minute
maps. Check your local Yellow Pages.
- Use a Global Positioning System (GPS) receiver.
- Call the local library or town hall. They'll often know their own latitude and longitude. If you don't live too far away from the library or town hall, that may be sufficient.
- Use the ARRL Locator. This Web page will let
you enter a city and state and will return the location-- including the
latitude/longitude and 4-digit grid square). This is sufficiently precise
for finding 4-digit grid squares but not for 6-digit grid squares.
- http://www.fcc.gov/mb/audio/bickel/DDDMMSS-decimal.html
A utility for converting between decimal degrees and degrees, minutes, and seconds. The page also has a link to convert between NAD83 / WGS84 coordinate system and the older NAD27 coordinate system used by the FCC for broadcast authorizations.
From Lat/Lon to Grid Square
Once you've determined your latitude and longitude, you can use that
information to find your grid square. Again, there are several ways to do
this.
PC Programs for Grid Locator Calculations
You can determine your grid square by entering your geographical coordinates into a computer program. Several such programs are available for the PC:
- grid.com--GRID
- gridloc.zip--GRIDLOC, which runs from gridloc.bas (gridloc.doc is also included in the ZIP file) to calculate Maidenhead grid square and coordinates
- grid.zip--Contains GRIDX, a BASIC program that calculates grid squares and Great Circle headings and grid.exe, a memory-resident pop-up grid locator calculator
Available from ARRL
ARRL Amateur Radio Map of North
America -- 27 x 39 inches. Large, colorful wall map features current geographic
detail and labels, grid squares, call sign prefixes, boundaries and more.
ARRL Grid Locator for North America
-- Size 11" x 17"
The ARRL World Grid Locator Atlas
-- Containing all 32,400 Maidenhead Locator Squares.
Page last modified: 11:32 AM, 07 Jul 2008 ET
Page author: hq@arrl.org
Copyright © 2008, American Radio Relay League, Inc. All Rights Reserved.
