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

| August 13, 2014

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.

mapdatumexamplesdots-1

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.

    GPSformat

    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:

mapdatumexamplessmaller-1

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

mapdatumexamples-1

This is from a United States USGS map:

mapdatumexamples-2

And this is from a French map:

mapdatumexamplessmaller-2

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:

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Category: Skills

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