A Primer on GPS Use for Mountain Biking

Self portrait of my armpits and Garmin GPS12

See also: Which GPS Receiver for Mountain Biking?

This article was written so that the reader can make full use of the GPS data contained on the KLMBH site as quickly as possible. It's not intended to be a comprehensive guide to using a GPS receiver. I've made specific reference to Garmin receivers because this is the brand that I'm familiar with. I expect that most other outdoor-use GPS receivers will work on the same principles and will have similar principal features.

What is it?

The Global Positioning System is a navigational system maintained by the US military comprising 24 satellites and numerous ground stations. These satellites communicate with a receiver, usually hand-held, to determine the receiver's position by triangulation. So, strictly speaking "GPS" refers to the whole system of satellites, ground stations and software operated by the US military; the handphone-like device that you stick onto your bike's handlebar is a GPS receiver. It's the best use of the US taxpayer's dollars (all 12 billion of them) that I can think of, and for that reason alone you should run to your nearest store to get one.

This site explains how GPS work more eloquently that I can ever hope to.

GPS and mountain biking

Accurate maps of trails in Malaysia are notoriously difficult to obtain, an unfortunate legacy of the Malayan Emergency. Today, you can buy 1:50,000 scale topographic maps of parts of Peninsular Malaysia from the Mapping and Survey Department. But these maps do not always indicate where biking trails exist, and often show trails that no longer exist. Using a GPS receiver, you can do your own mapping and share trail information with other trail users. The most spectacularly successful use of a GPS for mapping trails in Malaysia must be Pat's map of Bukit Kiara.

How to use the information on this site with a GPS: Manually entering waypoints

Of course, you'll have to read the manual that came with your unit. If you own a Garmin unit, Dale DePreist has an excellent on-line manual that betters Garmin's own. Once you've done so, you can check out one of the pages on this site that lists the trail route using latitude and longitude coordinates, such as the Sungai Dua trail.

The route table will look like this:

Route: Sungai Dua 38.8km out-and-back through steep and sometimes technical 4wd track
Waypoint	Coordinates	km	Directions and Remarks
D1	N3.26.753 E101.59.813	0.0	Trailhead
D2	N3.24.804 E101.58.262	6.8	River crossing (Sungai Dua Olak)
D3	N3.23.999 E101.57.297	NA	Orang asli huts. Temporary or semi-permanent in nature, so don't be surprised if you don't see them.
D4	N3.23.735 E101.57.125	10.4	Top of the climb. Also the watershed between 2 river systems.
D5	N3.23.310 E101.57.046	NA	Rotten chassis of old 4wd.
D6	N3.21.812 E101.57.536	15.0	Log bridge crossing over Sungai Rogol. Look for trail continuation on the left after log crossing. Not obvious.
D7	N3.20.776 E101.57.493	17.5	River crossing (Sungai Kerau, which flows southwards).
D8	N3.19.864 E101.57.473	19.4	Trail ends at the bank of Sungai Kerau. Park bike here and trek upstream (south) to waterfall, about 50m away.
D9	N3.19.796 E101.57.476	NA	Kerau waterfall. Retrace your steps to return to starting point.

Now, switch your GPS on. If your unit allows, set it to "gps off" or "demo mode" (eg on the newer Garmin eTrex units) or set it to "Simulator" (for the older units such as the Garmin GPS 12.) Doing so will save your batteries as it shuts off the receiver circuitry.

Then, save each of the waypoints in the table above into your GPS receiver. The simplest way to do this is:

1. Go to the main menu and select "waypoint".
2. Select New from the 'on page' menu.
3. Enter the new name and the lat/long entry. (Specify WGS 84 for the datum, lat/long for the coordinate system.)
4. Select an icon, if your unit allows.
5. Select done when you are finished.

Do this for all the waypoints in the table. Once you are done, take a print-out of the trail directions and route table with you on your ride. As you ride on the trail, you will pass each waypoint that you have saved. Used in conjunction with the written directions, you should be able to find your way around the trail.

More advanced users can save each set of waypoints for a particular trail as a "Route" in the GPS receiver. To learn how to do this, RTFM.

Uploading Waypoints into your GPS receiver

You can also upload a waypoint file into your unit from your computer. I'll bet there are countless ways to do this, but I'll only describe the use of a GPS application called OziExplorer. Because that's the application I've used for the waypoint files found on this site and, more pertinently, that's the only way that I know how.

What OziExplorer does is to allow me to download the data that I've acquired on my GPS to my computer, which I will then post onto the KLMBH website. Then you can download that data from this site and upload it into your GPS. Of course, it's also much more powerful than that; for example you can use OziExplorer to overlay the GPS data over a scanned map to generate your own trail map.

To use OziExplorer, you'll need, in addition to your GPS receiver:

1. A non-obsolete PC with a free serial port. (Lots of RAM will be nice if you want to use scanned maps.)
2. A cable connector that will allow you to link your GPS to that serial port. If you own an eTrex Venture/Vista/Legend, this should have been bundled together with your unit. If you don't have one, you can buy one from the supplier, or if you're an electronics whizz like Pat, you can make one for yourself and all your friends.

First, you need to install OziExplorer. A shareware, demo version of the application is downloadable from oziexplorer.com. The demo version has several functions dis-enabled, the most important of which is the ability to calibrate a scanned map using more than 2 points on the map.

You'll need to spend some time fiddling with the configuration settings, all of which are more than capably explained by the Help function in OziExplorer, so nothing will be gained by my repeating them here.

Now, download a waypoint file to your computer: try dua.zip. Uncompress this archive, which will contain two files: "dua.plt" and "dua.wpt". Then, all you need to do is go to the "Garmin" menu (or your brand of GPS) and select "Send Waypoints to GPS". Locate the .wpt file on your c: drive and you're off.

It's a bit of an effort to do all of the above just for the ability to automatically upload waypoints. Fortunately, OziExplorer is much more useful than just that, one of which is its ability to manage tracklogs.

Tracklogs

You'll notice that your GPS will map your route as you travel. These squiggly lines, called tracklogs, are stored as points in the memory of your GPS. The GPS receiver then uses some kind of software wizardry to join the dots on its display. But there's usually limited space for this data. On my very used Garmin GPS 12, there is space for 1024 points, while the newer eTrex units take 2048 points. (Woo hoo!)

Because of the limited space, this data will be overwritten (if you've set the "track" setting to "wrap" in your unit) once you've used more than the alloted space. But using an application like OziExplorer, you can save this tracklog file on your computer for later use. When you want to return to the trail in question, you can upload it into your GPS receiver in much the same way that you uploaded waypoints. You can also upload a tracklog that someone else has recorded.

Go to the "Garmin" menu (or your brand of GPS) and select "Send Track to GPS". Locate the .plt file on your c: drive (which you have downloaded and uncompressed earlier) and that's it. The tracklog will now be displayed in your GPS.

Your GPS unit will also be recording its own tracklog. It's important that you now turn off the tracklog recording on your unit. This will avoid a confused jumble of lines appearing on the display and, more importantly, will prevent the uploaded tracklog from being overwritten. (Note that on the earlier, basic Garmin eTrex unit (the yellow one), you can't turn the tracklog setting off. On the newer eTrex units (Vista/Venture/Legend) you can turn it off but it will resume recording if you turn the unit off and on again.)

To turn off the tracklog recording on a Garmin GPS receiver:

1. Go to the map page
2. Select OPT or CFG (depending on model)
3. Select "Track Setup"
4. Select the "Record" field
5. Select "Off"

You will now have a tracklog of the Sungai Dua trail displayed on the screen of your GPS. Your current position will be marked as a dot in the middle of the display. All you now need to do is to keep following the track. You should be able to tell fairly quickly (50-100 metres) if you've started to go off the correct trail, because the position marker will start to diverge from the marked track. Because you've turned the tracklog recording off, no other lines will be displayed on the screen, and the displayed tracklog will not be in danger of being overwritten.

Fancy stuff: Mapping

A valuable feature of OziExplorer is its ability to overlay GPS data onto a scanned map. A map may be imported from an image file (the demo version only takes bitmap files). You will then have to calibrate the image by identifying points on the map for which the coordinates are known. The registered version of the software allows you calibrate an image using 7 points, which will ensure a high degree of accuracy. The shareware version only allows you to calibrate using 2 points.

To load and calibrate a map, just follow the simple steps laid out in the help function of OziExplorer. Briefly:

1. Select the "File" menu, and then "Load and Calibrate Map"
2. Identify and select an image file
3. A dialogue box will appear. Select the tab marked "Point 1". The cursor will now be transformed into a cross-hair.
4. Click the cursor on a point on the map for which you know the coordinates. If the map has gridlines, you can simply select any suitable intersection of the lines. However, none of the Malaysian topographic maps that I have seen have latitude/longitude grids, although they do have lat/long marked along each side of the map. In such a case, you can do one of the following:
   • mark lines that are parallel to the lat/long markings on the map itself prior to scanning, or on the image file prior to loading it into OziExplorer. Click at the intersection of the lines to identify it as Point 1.
   • you can also calibrate an image on your computer monitor by using a square-edged piece of paper. Hold up the paper to your monitor, aligning one edge parallel to a latitude marking on the edge of the map and another edge to a longitude. Click at the corner of the edges, to identify it as Point 1.
5. Enter the coordinates into the fields provided.
6. Repeat for points 2-7.

Overlaying GPS data, in particular tracklogs, over a scanned topographic map allows you to update the maps with actual trail information. In the example at right, the tracklog of an actual ride on 11 August 2002 has been laid over a section of the map. Note that the map itself gave no indication of there being a trail where we cycled. Significant points along the trail (in this case 2 river crossings) that have been marked as waypoints in the GPS receiver can also be displayed in the map.

Which GPS to get?

Click here for an outline of which features to look out for, and which models will be suitable for mountain biking or hiking in the dense Malaysian jungles.

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