How your GPS can get you lost – and how you can prevent it

| August 13, 2014 | 0 Comments

OutdoorPeople-1Your handheld GPS receiver can give you your location with incredible accuracy. However, if you don’t understand how to set it up it could also get you lost.

In this article, we’ll look at the effect of selecting the wrong map datum in your GPS receiver’s settings and show you how to ensure that you don’t get lost because of it. You don’t need to understand what a map datum is to use your GPS receiver, so we’ll be steering clear of the technical theory.

How big a difference can it make?

If you choose the wrong map datum, you could be a long way from where you think you are. Using the following map as an example, your GPS receiver could tell you that you’re on a trail located at the bottom red dot when really you’re over 1km away at the top red dot. In this situation you would become utterly confused because your location on the map wouldn’t match what you see around you.


Prove it to yourself now

If you follow this exercise, it will show you how we arrived at the red dots on the map shown above, and help you to learn the importance of selecting the right map datum.

  1. Turn on your GPS receiver and wait to get a signal.
  2. Make sure it is set up to display a grid position format and not a longitude/latitude position format.
  3. Longitude/latitude formats start with hddd…

    For example, the Garmin eTrex Vista HCx has three longitude/latitude position formats shown as the first three rows in this screenshot.


    All the other choices are grid position formats.

    The sequence of button pushes used to change the grid position format varies by brand and model of GPS receiver. On a Garmin eTrex 10 it’s in the settings > Position Format page, and on a Garmin eTrex Vista it’s on the Setup > Units page.

  4. Draw a table like the one shown below and enter the position coordinates that your GPS is displaying on the main page.
  5. Datum First number Second number
     0594320  4130719
  6. In your settings, look at which map datum you currently have selected and write it in the table. In this example it’s WGS 84.
  7. GarminSetup

    Datum First number Second number
    WGS 84 0594320 4130719
  8. One-by-one, change the map datum that you have selected and write in the new coordinates that are displayed. The table below shows coordinates for seven example map datums.
  9. Datum First number Second number
    WGS 84 0674320 4430719
    Arc 1950 0674390 4431090
    Austria 0673856 4429630
    Guam 1963 0674274 4430698
    Indonesia 74 0674331 4430727
    Tokyo 0674681 4429513
    RT 90 0673873 4429649
  10. For the First number column find the largest number and subtract the smallest number.
  11. 0674681 – 0673856 = 825

  12. Repeat for the Second number column.
  13. 4430727 – 4429513 = 1214

What these numbers mean

The number pairs that you wrote down show your location in meters from a known reference point. The first number of the pair is your distance east of the reference point and the second number is your distance north of the reference point.

Subtracting the small number from the large number shows the variation in distance you can get when selecting a datum in your GPS receiver that’s different from the datum that your map uses.

In our example the distance varied by as much as 825 meters in an east-west direction and 1214 meters in a north-south direction. Now, this is not exactly correct because the example picks the biggest difference in each direction so it’s the worst possible case for each axis. Also, I wasn’t actually at the location shown in the map above and that map doesn’t use any of the datums in the example. But, it still gives you an idea of the magnitude of the distance that you could be off — hopefully enough to convince you that map datums are important.

At this point you might be thinking, “How can your position be different when you were sitting in the same place?” The answer is because each map datum uses a slightly different mathematical model for the shape of the earth and the grid will be placed on the map differently depending on which one is used. For you to locate your position accurately, your map and GPS receiver must be using the same mathematical model of the earth.

How do you know which datum to select?

Now that you know how important it is to select the correct map datum, you need to know how to find which datum to select.

If you have a decent topographic map, the datum will be written somewhere on the map. See if you can find the datum information for the following four maps.

This is from a US Tom Harrison map, my favorite brand of US map:


This is from a UK Ordnance Survey map that is written in English and Welsh:


This is from a United States USGS map:


And this is from a French map:


The answers are:

  1. NAD27, written as 1927 North American datum
  2. (OSGB) 1936 Datum, which would be selected on a Garmin receiver as British Grid
  3. NAD27 and NAD83. This is interesting because this type of map doesn’t have any grid lines on it. It has tick marks at the edge of the map so if you want a grid you have to draw it yourself. It actually has tick marks for two different map datums so you can pick the one you want to use.
  4. WGS84

You can see from these examples that it isn’t always obvious which datum to use. Someone new to navigation may see the words 1927 North American datum without knowing that this says the map was created using the datum NAD27. Becoming familiar with the names of the most commonly used datums will help a lot.

How to make sure you don’t get lost

Now we’ve arrived at the most important part of this article — how to make sure you don’t get lost because you have the wrong map datum selected. There are two steps:

  1. Before you leave your house for an outdoor adventure, review the map you will be using and set the datum on your GPS receiver to match.
  2. Verify your position at the trailhead.
  3. To verify your position at the trailhead, get the coordinates from your GPS receiver and plot them on your map. If the coordinates don’t place you where you think you are, either your selected map datum is wrong or you’re not where you think you are. To avoid getting lost, don’t move off until your map matches your receiver.

For more terrific articles about navigation, see:

If you enjoyed this article, why not subscribe to our free email updates list?

Top 10 navigation principles you need to know

| August 11, 2014 | 0 Comments

Here are 10 things you should know about navigation before heading out into the woods.

1. Your map, compass, and GPS receiver are complimentary tools

You can navigate with just a map, you can navigate with just a compass, and you can navigate with just a GPS receiver. However, each  has its strengths and weaknesses, and when you learn how to use them together you will increase your confidence in being outdoors.

At the most basic level, a map gives you route-finding information, a GPS receiver gives location information, and a compass gives you direction information. How they are used together will be covered in future articles on Urban Outdoor Warrior.

2. A paper map is still essential

Our smartphones, tablets, and phablets have almost become essential tools for navigating around our urban environments. They can display maps at a moment’s notice, and you can’t beat their convenience for finding that restaurant before your date gives up waiting for you. For fun, there are also countless GPS and compass apps to play with.

When you’re planning an outdoor adventure that will take you out of sight of familiar concrete, however, you’re playing a different game. If you get too away from populated areas, or you’re in a canyon, you’ll have no cell phone coverage and that means you won’t be able to download a map. Your smartphone’s GPS receiver will still give you your location, but that’s not much good without a map on which to display it.

If you’ve thought ahead, you may have downloaded a GPS app and some maps onto your phone before you set out. This puts you in a better position, and in a pinch they’ll help, but looking at a map on a phone is like looking at the world through a keyhole. That tiny piece of map that you see is going to limit your route-finding abilities and make your progress slow. And don’t even try to take a bearing off it with a compass!

3. There are two styles of navigation.

There are two main styles of land navigation and it will give you the most flexibility if you learn them both.

The first style assumes that your GPS receiver will always work, so you can forget about navigating while you’re on the move. You can go anywhere you want as long as you don’t move off the edge of your map. Your GPS receiver will give you your location whenever you need it and you can pinpoint your position on the map. This method is more casual and will almost always be sufficient.

The second style is for you if you:

  • Are more risk averse, as it assumes that your GPS receiver may not work
  • Don’t own a GPS receiver
  • Want to learn the traditional art of navigation and be the best that you can be in the outdoors

With this style, you track your location on the map at frequent intervals while you’re moving and familiarize yourself with the features around you. You know where you are at all times and only use your GPS receiver if you mess up and aren’t quite sure where you are.

4. The sun is a useful quick compass

You don’t need to be a boy scout and make a shadow stick to be able to use the sun for direction. You are your own shadow stick. If you live in the northern hemisphere, the sun is always in the southern sky. As your shadow falls opposite the sun, your own shadow will always point towards the northern half of the sky.

Everyone knows that the sun rises in the east and sets in the west. This means that in the morning the sun is south of due east, around noon it is around south, and in the evening it is south of due west. As your shadow is opposite, in the morning it points north of due west, around noon it points around north, and in the evening it points north of due east. It’s not the most accurate compass, but if you only need to make sure you’re heading in the right direction on a trail, it doesn’t need to be. For example, if you need to be hiking roughly north and your shadow is falling behind you, you’re going the wrong way.

5. Basic compasses and GPS receivers are enough

It’s more important to have a basic compass and basic GPS receiver that you know how to use than it is to have the latest fancy gear that does everything including confuse you. If you’re new to navigation start with the simplest (and cheaper) quality tools and learn the fundamentals of navigation. When you’ve got it covered and can no longer resist those cool additional features that will impress your friends, don’t hesitate to upgrade if your wallet can handle it.

6. The only thing you really need a GPS receiver for is…

The only thing you really need a GPS receiver for is to give you your grid location so you can find where you are on your map.

If you don’t have a map (please don’t let this be you), you’ll need to mark a waypoint at your car, or turn on the tracking feature. Then when you’re ready to head back to your car, use your GPS receiver to tell you how far away it is and in which direction you need to go. A GPS receiver with track recording turned on will always allow you to re-trace your steps and get back to where you started. This is great, but it may not always be as great as it sounds. It may be a 5-mile uphill climb to the water in your car but it may only be a five-minute hike to a creek with cold, delicious, running water. And remember that if you took a waypoint at your car, your GPS receiver will give you an accurate straight line direction back to your car. However, it won’t tell you about that cliff you’ll fall off if you follow that straight line. You’d better remember that map after all!

7. A baseplate compass is a compass and a protractor

If you don’t understand this you will never progress beyond rote memorization of the steps needed to use it.

To really understand how navigation with a compass works, it’s best to start with a non-baseplate compass for measuring and sighting bearings in the field and a separate protractor for measuring and drawing bearings on a map. Before long you’ll be saying, “Oh, that’s why you do that” as you go through the steps of using your baseplate compass.

8. Nearby magnetic objects will make your compass readings inaccurate

We all forget this from time to time, especially when we’re tired. Often we are just getting a rough bearing to make sure we’re not traveling the wrong way down a trail, but when accuracy counts this can make a difference.

It’s worth checking your gear before you head out to see which items affect your compass. It’s easy to do by putting your compass on a table and one-by-one moving metallic objects towards it noting at what point the needle starts to deflect. Start with your watch (one of the biggest culprits), and include glasses, pocket knives, phone, stove, etc. If that Rambo knife in the top of your pack is going to throw off your compass consider leaving it at home (in fact consider that even if it isn’t!). To be sure, you can always put your pack down and walk away a few yards before taking a reading, but if you’re in bear country don’t be surprised to find that it’s not there anymore after taking that reading.

Also, make sure you’re not taking a reading anywhere near your car or electrical pylons.

9. You have to choose the right map datum

Map datums are one of modern navigation’s little technical annoyances, but you can’t ignore them. We don’t need to get into the theory here, you just need to know that if you don’t choose the datum on your GPS receiver to match your map, your position on the map may be off by  hundreds of meters, enough to confuse you and send you up the wrong hill.

You can learn more here about how your GPS can get you lost because of the wrong map datum.

10. Yes, you do need to understand declination

Declination, also known as magnetic variation, is the hardest concept in navigation but, unless you live in an area where true north and magnetic north point in the same direction, or if you’re sure you’re going to be staying on major trails and only need a rough direction from your compass, you do need to understand it. This is not the time to teach declination, we’ll just leave it by saying it’s worth investing some time to understand it.

That’s it. There are sure to be different opinions about the 10 most important things to know about navigation. Leave us a comment to tell us what you think.

For more awesome articles about navigation, see:

If you enjoyed this article, why not subscribe to our free email updates list?