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January 15, 2007

New blog for GPS users

Filed under: GPS — mtbguru @ 11:40 pm

Free GeoTools is a new blog that focuses on tools, tips and tricks that can make life of GPS users easier… without making them spend big bucks. Something that we here at MTBGuru also strive for.

Btw, this prompted me to update this blog’s link lists in the sidebar – I’m planning to add more in this space soon.

GPS markets and trends

Filed under: GPS — mtbguru @ 10:05 pm

Some interesting reading material on GPS markets and trends:

This U.S.News article talks in great detail about Garmin, its history and future – possibly a quite bright future. An interesting bit from this is that worldwide TomTom and Garmin are currently shipping about the same number of units. And more competition is on the horizon (besides a number of smaller companies, also giants such as Sony are stepping into the GPS market, to be followed perhaps by smartphone producers such as Motorola and the likes, or even Apple). This is probably good news for users, who I’m sure wouldn’t mind seeing some of the quite hefty prices drop.
(via GPS Tracklog)

And this Yahoo News article mentions how bluetooth chip producer CSR has acquired two smaller GPS IC design companies, a move that could indicate we’ll start seeing more and more GPS functionality in various mobile gadgets.

January 10, 2007

Uploading trip data to your GPS

Filed under: GPS,Howtos / tips / tricks — mtbguru @ 10:51 pm

In a previous article we talked about some tools to download data from your GPS unit and save into GPX files. Now we’d like to outline what you can do with GPX files you’ll find on MTBGuru (which aren’t necessarily yours).

For each public trip on MTBGuru that has GPS data, you can download the associated GPX file (using the ‘Download GPX file’ link in the sidebar) as well as a Google Earth (KML) file. By browsing around the map, using the search box or by clicking the ‘Recently Added Trips‘ link you may find a trip or ride that catches your interest. Say you want to do this ride yourself: the trip summary gives you an idea of what to expect in terms of distance and elevation gain and if there’s timing info you’ll know even better what you’re up for. You can now use the GPX file to send the data to your GPS unit, so you’ll be able to retrace the given route.

Some units allow a straight upload of the entire GPX file with its waypoints and tracks. In some cases though, there are complications:

  • For instance, on some Garmin units (e.g. the Etrex family) the size of tracks you can upload is limited to 500 points (even though they can *record* tracks of much larger size) – larger tracks will be truncated
  • Some units don’t allow upload of tracks, only waypoints and/or routes

A quick GPS terminology 101 may be in place here:
A ‘route’ is a sequence of waypoints that indicates a certain route. A ‘waypoint’ is basically a set of three numbers (latitude, longitude, elevation) indicating a point in space, usually on the surface of the planet ;) . A track is a sequence of (recorded) waypoints but generally also contains time information.

GPX files you download from public trips on MTBGuru contain tracks. The time info in this file is stripped but all spatial information (long,lat,ele) from the trip is available. Only trip owners have access to the time info in the GPX – using the ‘My GPS Files’ link in the sidebar they can always retrieve the original uploaded data.

As mentioned before, uploading these files directly to your unit can sometimes result in truncated data and other problems. However, you generally don’t need that many points to navigate and retrace a given trip. You can use a host of software tools (e.g. Garmin’s MapSource on PC, Routebuddy on Mac OS X) to create routes with a limited number of points from tracks and upload these instead.

GPSBabel filter

Our favorite tool, GPSBabel, has a very neat and simple way to achieve the same: by using its ‘filters’ you can automatically reduce the amount of points in a GPX file. Select your GPX file as input file and set up your filter as shown in the screenshot (using distance between the points as filter); the output file will be a GPX file with a reduced number of points. You can change the amount of points by changing the merge distance between points.

January 6, 2007

GPS download survival guide

Filed under: GPS,Howtos / tips / tricks — mtbguru @ 12:37 pm

GPX, used by MTBGuru, is a great and open standard format for GPS data – not all GPS manufacturers make it easy to download your data in this format though, so we thought a brief GPS download survival guide was on its place. If the software that came with your unit doesn’t support saving in GPX, this post is for you – and even if it does you may discover some interesting new tools here.

If you don’t feel like reading the whole post, one word: GPSBabel – Robert Lipe’s tool runs on about every OS, is free and totally awesome, use it and donate to support the project.

GPSBabel can be used to read, write and convert GPS data in an entire range of formats that are in existence. It can directly interface Garmin and Magellan units using the USB or serial port on your computer. It also understands TomTom and Suunto file formats. And so on and on, check here for a list of supported formats.

It’s a console mode tool but has nice graphical front-ends in both Windows and Mac OS X. Below is a screenshot of the front-end for OS X called GPSBabel+, written by Karl Smith (and can be downloaded here). I use it to download data from my Garmin Edge straight into GPX files on my Mac iBook.

GPSBabel+ (for OS X)

Hook up the GPS to the USB port, set up the input and output options as shown in the screenshot, and press ‘Save File’, and you get all data on your unit in a GPX file.
The GUI front-end on Windows works similarly.

Another very cool software tool on Mac OS X is LoadMyTracks (screenshot below), which is available as a free beta version. It simply does what its name says, and you can save into GPX or Google Earth’s KML format. Besides Garmin and Magellan, also TomTom devices can be hooked up directly.

LoadMyTracks (for OS X)

On Windows, a very nice and free program is Sporttracks – you can use it to download data directly from GPS and save into GPX.

A good web application to convert GPS data is GPSVisualizer. It uses GPSBabel as its conversion engine.

Undoubtedly, there are many other software tools and programs out there that can do the job – I focused here on a few free or inexpensive tools that I think work very well. If you know of other and/or better tools, feel free to mention it in the comments.

Let me wrap up by saying that I’m a bit bummed about what Garmin is doing in this area: MapSource on one hand (that comes a.o. with the Etrex series of GPS units) allows you to export your data into GPX without any trouble but in Training Center (that comes with the Edge and Forerunner series) it’s a real headache to get your data out. Granted, .hst and .crs are open xml formats, containing additional information such as heart rate etc, and with some effort it’s not too hard to convert these, but why make us go through the trouble?

January 4, 2007

Multiple tracks in GPX files

Filed under: GPS,Howtos / tips / tricks — mtbguru @ 1:59 am

When a GPX file is uploaded to MTBGuru, the file is examined and probed for GPS tracks. GPX files may contain single or multiple tracks.

When you look at the file in a text (or xml) editor you can identify the tracks as content delimited by <trk> and </trk> tags. And tracks themselves may consist of a number of ‘track segments’; segments are identified by the <trkseg> and </trkseg> tags.

So how does MTBGuru interpret these files? Our goal was to make it as easy as possible to fetch the data for your trip, and hence we needed to be quite flexible, as different GPS units will store their data differently. Even using a single device there are many different ways or tools to download the data, each potentially resulting in differently structured GPX files.

In the simplest case, the GPX file contains only a single track – nothing special is going on then, the trip is created using the data in this single track. Track segments are concatenated and displayed as a single trace on the map.

But what happens when multiple tracks are present? When data is downloaded from the GPS, often times it will come as a single file containing all the tracklogs stored in the memory of the device, converted into a single GPX file. This GPX file will typically contain a number of different tracks representing the various tracklogs you’ve recorded over time.


As you may have found out, there are two ways to create a trip from GPS data in MTBGuru (see figure):

  • By Upload GPS Data: after you upload a GPX file, you are presented a screen displaying all tracks in the file (see screenshot 1 below), as well as information such as the number of points and date and time. You can then select an individual track and base a trip on it by clicking Create trip.
  • By Add a Trip: here you start by creating the trip first, and then attach a GPX file to it using Upload GPX file. If your GPX file has multiple tracks, you’ll be shown a different screen now, that allows you to select and combine multiple tracks (using checkboxes) for this one trip (see screenshot 2 below).

The second option is very useful in cases where the GPS unit was for instance suffering intermittent reception during a trip. What typically happens then is that the tracklog in the memory of the device is broken up into different tracks, with ‘gaps’ in between them. You can now easily stitch everything back together by selecting the relevant tracks using the checkboxes – the time and distance gaps between the different tracks are indicated and help you decide whether the tracks belong together. The combined set of tracks is displayed as a single trace on the map – the gaps may or may not show up as visible artifacts, depending on the amount of reception loss.

The first option is preferable if your GPX file contains data from various trips or rides, from which you’d like to select and create individual trips.

P.S. GPX files may also contain other data types such as waypoints and we do anticipate support for the latter – currently all waypoints in GPX uploads are stored in our database, such that they can be accessed and used in the future.

Screenshot 1, after uploading a multitrack GPX using Upload GPS Data

Screenshot 2, after uploading another multitrack GPX file using Add a Trip
GPS tracks screenshot2

December 23, 2006

Odometer versus GPS

Filed under: General MTBGuru stuff,GPS — mtbguru @ 9:11 am

In the previous post we compared GPS elevation data to elevation data from topographic maps – the latter giving you generally more averaging than the former. Something similar is going on when you compare GPS distance data to bike odometer distance data – but the roles are inverted here: the GPS averages more than the odometer.
Let’s look again at the snippet of Garmin Edge 305 data from the Tahoe Rim Trail:

GPS data

The distance between a given datapoint and the previous one is now plotted in blue. The GPS device records longitude and latitude, from which this distance can be calculated using fairly simple trigonometry; we use the haversine form of the great circle distance, which works well (in terms of rounding errors) for points relatively close to each other.

Let’s now consider the data you would get from an odometer. First, we assume that the circumference of the wheel is properly set up in the odometer, in which case it can be considered quite accurate. The odometer essentially takes a datapoint for every revolution of the wheel; for a 26 inch wheel and a speed of 5 mph (hey, it was a tough technical climb at altitude), there is about 1 second in between datapoints. For higher speeds, there is less than that.

Compare this to the GPS: the Edge at its fastest takes 1 point per second, and usually much less, as you mostly want to use it in its ‘smart’ or adaptive mode (to limit the amount of data recorded and make economic use of memory). On the graph below, this is illustrated by the higher density of (red) odometer datapoints than the GPS points:

GPS + odo data

The odometer of course doesn’t really record all these points, but the figure shows what they would look like if it did. It does record a cumulative (total) distance number though. How does it get this? By simple adding the lengths of all line segments between the points. This is done for both GPS and odometer in the figure below. The odometer will almost always give you a higher total distance reading than the GPS (in particular on fairly bumpy or technical rides) because all the red (odometer) line segments add up to a higher total than all blue (GPS) segments – this is so because the odometer simply samples at a higher rate than the GPS (see also the ‘length of a coastline’ problem). You could of course try to compensate for this effect by doing lots of wheelies ;) .

Update (12/30/06): as ragetty observes in the comments below, the latter paragraph isn’t entirely accurate: the odometer doesn’t give a better number because it samples at a higher rate than the GPS, but because the wheel tracks all features of the trail (at least, those features with size in the order of the wheel diameter). What I tried to say is that if the GPS would sample at a higher speed, it would be in better agreement with the odometer.

GPS + odo data 2

December 17, 2006

Elevation accuracy

Filed under: General MTBGuru stuff,GPS — mtbguru @ 8:51 am

One of the perennial points of discussion when talking about GPS elevation profiles is the accuracy of the elevation measurement. Total elevation gain and loss on a given route can significantly deviate between various devices and from what you would calculate on a topographic map.

On our FAQ page we try to delve into this:

Most GPS devices use either the GPS signal (using triangulation), a built-in barometric altimeter or a combination of both to give you an elevation reading. The latter will generally give you the most accurate results, but unfortunately in many cases there will still be significant errors – for instance, the barometric pressure may change during your ride for other reasons than elevation change (i.e. weather changes), or the sensor itself can get out of calibration.

That being said, the devices with barometric sensors will usually give you reasonable results. There is however a problem that typically occurs on technical trails: all the tiny ups and downs of the trail result in minute elevation changes that are being recorded. These accumulate and on longer rides it will generally result in overestimates for the elevation change numbers compared to what you’ll see on a topographic map.

Accuracy is generally measured with respect to what your route mapped on a high resolution topographic map would read – this can be obtained by downloading your GPS track data and mapping the track on for instance National Geographic’s Topo! tool. Of course, the cumulative elevation numbers you will get this way are depending on the spatial resolution of the topographic elevation data – effectively resulting in some averaging. which numbers are best is in a way a mattter of taste: the topo value gives you an averaged and ‘reasonable’ looking number, but on the other hand, you did ride all those tiny bumps up and down (the problem statement is somewhat similar to the ‘length of a coastline’ problem).

We chose to apply some averaging in calculating the total elevation gain and loss from the GPS track data, in order to get fair agreement with values that are typically obtained using topographic data.

The best you get with a barometric sensor is probably a relative accuracy of half a meter or so. ‘Relative’, because in order to obtain absolute accuracy you would certainly need to recalibrate the sensor frequently.

The figure below shows some elevation data recorded on the Tahoe Rim Trail (fairly technical) using a Garmin Edge 305 in the ‘smart recording’ setting (in this setting, the sampling speed is adaptive), and it illustrates some of the minute, sub 1m elevation changes mentioned before.

GPS elevation data

December 4, 2006

Save the planet, get a GPS!

Filed under: GPS — mtbguru @ 6:47 pm

If, besides to enjoy MTBGuru.com ;) , you needed another reason to buy a GPS unit, check out this post on GPS Tracklog:
You’ll be saving fuel and reducing your emissions by 7%!

This is mainly a result of a reduction in driving (if you actually use the GPS for driving, of course), but in addition I guess you’ll print out fewer online maps (less dead trees), and if you’re using it for outdoor endeavours: think of all the fuel and emissions you’ll be saving the rescue crews by not getting lost out in the wilderness anymore!

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