Bushwalking Navigation | How far have I walked?

Want to know how far you have walked or how far you still have to go? Which  tools can use to find this info? What pre-walk planning should you have done? What should you be doing while you are walking? Can your GPS be relied upon to give this information?

 Part 1 of this article was published separately but has now been merged with Part 2

Knowing how far you have walked can determine whether you have a safe trip. Without knowing the distance you have walked, and the time it has taken, you can't estimate how long it will take you to get to your camp site and whether you will make it before nightfall.

The best way is to "thumb" your map as you walk along, so you know where you are at all times and use this information, along with your route plan, your average speed and time since starting, to work out how far you have walked. Your GPS can give you a good guide too, but over long distances your GPS can be significantly inaccurate and should not be relied upon as your sole source of this information, especially if you are moving slowly in difficult terrain.

Before you can work out how far you have walked, either you or your GPS need two bits of essential information

1. where you started
2. where you are now

The first should be easy, but the second can be much more difficult. You can make it easier for yourself by doing some preparation before you leave home and again at the start point (trailhead for those from NA).

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Before you leave home

• purchase or download (lucky NZanders!) a map and guidebook if available.
• use navigation software (smartphone or PC*) to explore your digital map, measure distances, select routes and waypoints
• view the terrain in Google Earth or Google maps, select your waypoints, route and measure distances
• enter way points into your GPS and link them as a route, using the software or just manually
• measure distances using a map measurer or piece of cotton or a ruler, depending on whether the track is straight or windy
• prepare a route plan
• mark your map with your route and waypoints

* I have used MacGPS Pro for the last 10-15 years on my Mac.

At the beginning of your walk

• locate yourself on the map
• check the time and start you stopwatch/timer
• mark the start waypoint on your GPS and set it to navigate to your  next planned stop
• reset the Trip Odometer on your GPS
• reset your pedometer if you are carrying one

PS don't forget to adjust the time and date on your camera as well, as it is good to be able to match photos with your GPS location when you get back home.

While you are walking

• "thumb" your map, reading map-to-ground as you walk along, "ticking off" prominent features as you walk along
• use your GPS, and known features, to work out your average speed and use your stopwatch, which you started at the beginning of the leg, to estimate how far you have walked, based on the average speed
• continually check that the terrain matches where you should be on the map based on your average speed and time you have been walking, less breaks. This is a very difficult skill to learn and needs continual practice as it is very easy to miss a creek junction or mistake a knoll. Carry a map even on an easy day walk and practise.

When you reach your objective

• your objective should be a prominent feature which is on your map and your route plan. Use navigation techniques such as aiming off, handrails, catching features and attack points to help you locate the feature.
• use your GPS to check your location, making sure you are using the correct map datum
• use your route plan to look-up the distance, which you hopefully measured using one of the devices listed above before you set off
• use your GPS's Trip Odometer to tell you how far you have walked (subject to inaccuracies: read below)

Why my GPS can't measure distance travelled accurately.

Your GPS trip odometer should only be used as a backup in determining distance, not as your primary device, as it is unable to accurately determine distance travelled in typical off track walking terrain, as occurs when you are walking along a windy creek track with tree cover, overhanging cliffs and the odd waterfall to climb. For most use on open tracks the accuracy will be adequate but for a 10 km walk leg through difficult terrain, the inaccuracy could be as much as 10-25% of  the true distance.(Source: Garmin Forums)

One of the reasons for this is that the GPS adds the distances, as the "crow flys", between the points it has saved, to the trip odometer and these will usually be slightly shorter that the actual distance you walked, especially if you have changed the data logging from the default which is usually once per second (1hz) to something less frequent. The only exception to this will be if you are walking in a straight line when it should be able to measure with 100% accuracy. The inaccuracy will be further increased if you make frequent stops, turn frequently or walk slowly, say less than 3.5 km/hr, so that the GPS doesn't know you are walking. Walking up a steep incline can also produce inaccuracies, as the GPS only measures horizontal distances.

The accuracy will be increased by increasing the data logging rate  eg 1Hz (once a second) to 5Hz. The problem with this is that your battery may not last as long and when you reach the internal memory limit it may start deleting the oldest.

From the Garmin Forums: Distance of Trip odometer not the same than distance of Track

When you walk (or drive), the GPS is constantly doing calculations based on where you are this instant compared to where you were on the previous location reading so it "knows" which direction you are traveling and how fast you are traveling.

At each of these calculations, it also calculates the distance traveled and adds that to the Trip Odometer.

Most GPS units do these calculations approximately once each second. You do not travel very far in one second, even in a motor vehicle traveling at the speed limit, so each of these calculated distances will be fairly accurate. That means when you add them all up, as the Trip Odometer is doing, the total distance in the Trip Odometer at the end of the trip will be fairly accurate.

One thing that will affect the accuracy of the Trip Odometer when you are hiking is if you are not moving fast enough for the GPS to detect that you are moving. That could cause little pieces of travel to not get added into the Trip Odometer and the distance it reports to be shorter than reality.

Track files are different. If you save the track log to a file, it always prunes the log to 500 track points, regardless how many points there are in the raw log file. If your driving or hiking was not in a long straight line, you will "lose" distance when pruning the points. That is, if you walked in a curve that originally had 20 points marked and the curve gets pruned to, say, 3 or 8 points to describe it in the track file, you will not get the full distance of the curve calculated in the track.

That is because the calculations of distance in the track file all assume that the distance between each recorded point is a straight line. If you describe a curve with fewer points it will always look like the curve covers less distance.

If you transfer the raw track log file to Mapsource or Basecamp, you should get all of the track points and that should cause less of a difference between the track and the related Trip Odometer reading.

Further reading:

Bushwalking Navigation | Using Topo50 Maps (LINZ) for Tramping in New Zealand
Bushwalking Navigation | How to Choose the Best iPhone GPS App
Bushwalking Navigation: The Importance of Using the Correct Geodetic Map Datum.
Bushwalking Navigation | A Route Plan Workflow
Silva Map Measurer Plus
Bushwalking Photography Workflow | Share the Best of a Group's Photos Using iPhoto

Read more Navigation posts


  This article by Bush Walker is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.