Tuesday, October 25, 2011

Map and GPS Best Practices

The following are my personal opinions and recommendations and do not represent the views, opinions, or position of the BFRO.  

NOTE: Added new PDF version of a more in-depth look at this subject: http://home.comcast.net/~k.burns1969/Documents/Map%20and%20GPS%20Fundamentals.pdf

Handheld GPS units are critical pieces of equipment for use on BFRO expeditions for the purposes of documentation, land navigation, and personal safety. Not knowing where you are at 3am on a forest trail or road can bring about panic pretty quickly. I believe at minimum each group should carry a GPS and understand the basics of it's use which I will explain here.

Getting the most out of a GPS does not have to be complicated. There are only a few basic concepts which should be understood.

1) Track Logs

Almost all GPS units have the concept of a track log. The GPS simply records a continuous track log of it's location while it is turned on.When heading out to a new location for scouting or night manuvurers the current track log should be saved and reset so that a brand new track log is created.

Track logs are especially useful during night maneuvers since you might be following some cars to a location in the forest you have not yet been to. At the end of the night you simply reverse your path on the saved track to get back home. This particular feature has saved me more than once in the forest at night.

2) Waypoints

Waypoints are simply saved coordinates of a single location. It is a good idea to always save the following coordinates:

  1. Basecamp
  2. Significant Finds During Daytime Scouting
  3. Night-Time Stake Out Locations
  4. Where sounds or other encounters occurred.

Upon arriving at basecamp on day #1 of an expedition I delete all stored waypoints unless I have already pre-programmed ones in back at home for the purposes of the trip. During the daytime I simply use the standard GPS naming convention of 001, 002, etc and then record the meanings of the numbers on a small notepad.

  • 001 - strange stick thing
  • 002 - large pond
  • 003 - something else.

I will then transfer these coordinates into a larger notebook and note them on a paper map when I get back to basecamp. When I am out on night maneuvers I use a very simple notation to name the GPS coordinates.


Where n means the waypoint was taken at night. x is the day of the expedition. m stands for maneuver and y is for the waypoint number.

So a waypoint of "n1m1" means "first waypoint for location of night time maneuvers on the first night". If we leave the first location and move to another one the waypoint name would be "n1m2". The second night would have a format of "n2m1" and so on. I do not have a modern GPS with a touch screen so I am limited to a simple click stick in spelling out waypoint names. This naming system allows you to input a useable name into the waypoint description without typing out a long complex description. It is also flexible in that instead of n it would be d for day or instead of m it would be c for camp. I also use a similar naming notation on my digital audio recorder.

3) Maps

Most GPS's have the ability to show you maps of the surrounding terrain and while they are useful for knowing where you are on a map they are not useful for seeing and evaluating large amounts of terrain at once. The best maps to get are maps on a scale of 1:24000. This means that 1 unit of measurement on the map equals 24,000 units of measurement on the ground. So one inch on the map equals 24,000 inches on the ground. 1:24 scale maps are the only scale that is really useful in expeditions. Larger scales just do not show the details required to make decisions on where to perform field operations.

The best physical maps to have are USGS 7.5 minute quadrangles.  They are the smallest map units sold by the USGS. To order them you need to know what quadranagle you want. I have found two good sites for determining where terrain exists in the quadrangle system.

The first is the USGS map locator:


The second is Digital Data Services.


Digital Data Services actually allows you to download tif files of each quadrangle. Both sell paper maps of each quad.

For expedition use it is best to have the actual large format USGS quadrangle maps of the locations you are interested in. 

4) The UTM Grid System

Regardless of where you get your phyiscal maps they should always have the UTM grid system as well as the standard lat/long coordinate system.  Lat/long is an angular coordinate system which is not based on distances. UTM is a system based on distance and is far more suitable for land navigation purposes. The UTM grid system is a simple overlay of boxes in which each box equals so many square meters of space. This makes it very easy on a map with UTM tics to tell how far apart things are.

The UTM grid system seen on the map segment below illustrates the distance between tics as being 1000 meters. Each UTM grid box on a standard 7.5 quad map is 1000 square meters per box. This makes measuring distance between two points on the map relatively easy.

In order to take advantage of the UTM grid system your GPS must be configured correctly to correspond with the correct map datum and position format.  7.5 minute maps typically have a map datum of either NAD27, NAD83, or WGS84. If you aren't sure just look at the map and it will typically tell you on the bottom somewhere.

The position format on the GPS should be set to UTM rather than any lat/long. You will know the GPS is configured correctly when the coordinates look something like this.

13 S   471660m E   4329948m N 

To find this coordinate on a map you start by finding the Easting coordinate on the bottom left of the map. Move right until you find what 1000 meter grid it lies in. Then match up the Northing coordinate to find out where both lines intersect. A tool called an UTM roamer is useful for this but a ruler or just eyeballing it works just as good. Just remember to read right and then up (right = easting, up = northing).

Having everyone utilize a single map datum and position system guarantees that issues will not arise during the expedition with respect to where someone or something is located. UTM coordinates are also much easier to transmit over the radio since there is no discrepancy of what format you are using when the GPS is configured to use UTM positions. There are three different formats lat/long coordinates can be expressed in. This can be confusing if one of the parties does not understand the difference. 

5) Compass Use

GPS units are a great tool to determine where you are but are often over kill when you need to follow a simple straight line or bearing between two points. In that case a compass is often a better choice. It is important to always have the compass configured with the correct magnetic declination. Declination is the amount of drift from true north that your compass will point. Unless your compass is set for the correct magnetic declination than it is actually not pointing to the same direction of north that your map is Depending on where you are in the country you will have to adjust your compass for a negative declination or a positive one.

The NOAA has a website which gives you the current magnetic declination based on location.


Denver Colorado has a current magnetic declination of  8 degrees east. This means that any compass in Denver will show north as 8 degrees further east than the maps version of north.  Baltimore Maryland has a current magnetic declination of 11 degrees west. This means that a compass in Baltimore will show north as being 11 degrees farther west than the maps version of north.

The following pictures shows the bearing indicators of a compass adjusted for 8 degrees of easterly declination.

When using a compass to navigate over long distances (something you probably won't be doing on an expedition) it is important to have the map and the compass both agree with respect to north. Since you can't adjust the map you have to adjust the compass. A good compass will have a small screw that allows rotation of the heading dial to allow the compensation for magnetic declination. For a compass in Colorado, the heading dial will be turned right 8 degrees. For a compass in Maryland, the heading dial will be turned left 11 degrees. A good compass will have magnetic declination settings along the bottom of the dial to make this adjustment foolproof. This only needs to be done on a compass. A GPS should always be configured for True North since it needs to match the map. If you do not have a compass that can be adjusted for magnetic declination and you plan on using it to follow bearings on your GPS than your GPS should be set to Magnetic North. This is not really optimal since magnetic north changes year to year and your GPS might not be up to date with respect to what it thinks the current value of magnetic declination is unless of course it allows the value to be entered manually. Another option is to simply add or subtract the declination from the compass if it cannot be set manually. As a best practice you should always keep your GPS set to True North so that it matches the map. Chances are you will not be performing long distance compass navigation where magnetic declination will be a factor.

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