GPS Tracklog

GPS myths

GPSReview.net lists 10 myths about GPS. I've covered a number of these in detail in the following posts:

• Can my GPS be used to track me?
• GPS for your car: Factory-installed or aftermarket?
• SiRFstar III - Why it's a big deal
• Does heavy cloud cover affect GPS reception?
• Flying with GPS on commercial airliners

Why have a barometric altimeter?

Quite simply, the primary reason you would want a barometric altimeter in a handheld GPS is for more accurate elevation readings. This is especially useful for bikers, hikers and atheletes in training. You can use the resulting data to track your progress and maintain an accurate record of your trips. A barometric altimeter can also help you accurately place your location on a topo map.

Furthermore, you can use the altimeter to forecast weather trends. A falling barometer can indicate worsening weather. See the barometer section of this review for some really cool things you can do with your GPS!

GPS-based altitude error is generally worse than latitude/longitude error. A good explanation of why barometric altimeters are better than GPS-based elevation data comes from GPSInformation.net. Here is another good resource -- an electronic compass and barometric altimeter FAQ from Garmin.

Finally, a barometric altimeter is often paired with an electronic compass in higher end handheld GPS receivers.

GPS for your car: Factory-installed or aftermarket unit?

GPS navigation systems for cars are becoming quite popular, and there are some basic questions that  prospective buyers are asking. One is, should I buy a factory-installed GPS, or an add-on (also known as aftermarket or OEM) unit? Let's look at the arguments for and against each approach:

Factory installed GPS

Pros

• A better appearance, as units are built into the dash or located on the instrument panel.
• Theft is generally not an issue (unless they take the whole car!).

Cons

• The cost is typically considerably more than an aftermarket unit.
• Getting updated maps may be difficult and expensive.
• Fewer options are available.
• Units cannot be moved from one vehicle to another.

Why have an electronic compass?

A GPS receiver discerns your position on the surface of the earth by measuring the length of time it takes to receive signals from satellites. It cannot, however, tell what direction you are facing while standing still (or moving slowly). So if you are navigating to a waypoint, your GPS can only point you in the correct direction once you are moving.

This is where an electronic compass comes in handy. It can tell what direction you are facing, even while standing still. Not only does this make navigating easier, it also helps with projecting waypoints and orienting paper maps.

Electronic compasses will shorten battery life somewhat. They are typically paired with a barometric altimeter.

 

SiRFstarIII - Why it's a big deal

There are a lot of hot new GPS receivers that use the SiRFstar III chipset, but unless you're a heavy-duty GPS aficionado, you may be wondering, what the heck is this thing and what is the big deal? Simply put, it is a GPS receiver chipset. This is the little piece of silicon responsible for receiving GPS signals and passing the coordinate information along to a miniature computer in your GPS unit.

Why is it a big deal? Fast acquisition times and processing power. This translates into less likelihood of lost signals under canopy and in urban (or natural) canyons. In canyon-like environments, your GPS receives signals directly from satellites whenever it can, but it also receives reflected signals, bouncing off hard surfaces (multipath reflection). Your GPS determines your position by calculating how long it takes the satellite signal to reach your receiver, so reflected signals are a source of error. The SiRFstar III chipset has the processing power to do a lot of "what if's" with these reflected signals. This processing power also means that the chipset can consider weak signals that were ignored by previous chipsets. The SiRFstar III also has faster acquisition times, making for a faster time to first fix (TTFF), and a quicker reacquisition if the signal is lost.

All of this translates into amazing reception, under canopy, in urban centers and even indoors. And that is why the new Garmin units with this chipset are so hot.

Can my GPS be used to track me?

This has got to be one of the most frequently asked questions about GPS. The short, simple answer is no, your GPS cannot be used to track you. That is because most GPS devices are receivers; they transmit nothing. There are rare exceptions, such as the Garmin Rino series, designed to transmit your location to a friend.

Having said that, there is a more complex answer. First of all, if you have the tracklog enabled on your GPS receiver, someone with access to your GPS can physically connect it to a computer, download the track and see where you've been and when. Of course, that is all very different than real-time tracking. Just don't use a GPS to guide you to a bank heist!

Many companies do make real-time GPS tracking devices, which have transmitting capabilities built into them. These can be used to track everything from lost pets to teenagers to commercial vehicle fleets. We're not talking about your typical GPS receiver here; these are specialized devices.

And then there are cell phones with A-GPS. Many cell phones have options allowing you set it to "location on" or "911 only." You would need to have it set to "location on" to utilize GPS-assisted location based services. The legal issues surrounding access to this information by law enforcement agencies have yet to be fully sorted out.

Then there are other places a GPS may lurk, many times unknown to a person driving a car with GPS. Got Onstar? You've got GPS. Ever rent a car? Better check the fine print in the contract.

A couple of other references:

• Wikipedia entry for GPS tracking, which hypothesizes "The consumer electronics market was quick to offer remedies (radar detectors) to radar guns; a similar market may exist for devices to counter satellite tracking devices. Radio jamming of the relevant GPS or cell phone frequencies would be an option, as would a device which could detect the RF emissions of the GPS receiver circuitry."
• An article from Pocket GPS World on privacy concerns and the future of GPS.

Technorati tags: GPS

How are Garmin and Magellan topo maps different?

Over at Groundspeak, Geocaching.com's message forum, Peter gave a great explanation of the differences between Garmin and Magellan topo maps:

"As Sputnik indicated, the approaches taken by Magellan and Garmin are different. Garmin uses Digital Raster Graphics (DRG) to basically create an image of the USGS 1:100,000 series maps. Therefore their maps have all the detail and accuracy provided by those maps, and nothing else - which means they lack most road names and anything built since the maps were last updated (20 years or so ago in many areas). But they do show the features included by the USGS: springs, fire roads, quite a few trails, etc. Also note that the 1:100,000 maps are metric so the contour spacings are based on rounded numbers of meters. When your GPS is set to feet the spacings look a bit strange (e.g. contours at 164', 328', 492', etc.)

Magellan's topo maps are instead based on the Digital Elevation Model (DEM) technology where elevations are sampled in a grid pattern and the contour lines are then recreated by interpolation between grid points. That provides the contour line detail but by itself wouldn't give you anything else. So Magellan combines this topology data with the same road data they use for their street maps (based on TIGER for the original Streets and Topo programs, based on NavTeq for DirectRoute and 3DTopo).

If you want to get by on just a single set of maps then Magellan's approach is clearly superior. But anyone who wants auto-routing and elevation data (both good things to have) will need to get both sets of maps anyway. In that case each approach has its good and bad sides. Magellan lets you see all the data at one time on your screen since their street data is identical on both and therefore doesn't cause a problem, but they lack many of the older dirt roads and trails that are included on the old USGS maps used by Garmin. And, as Sputnik said, you can load both sets of maps in your Garmin and toggle between them. I frequently use CitySelect to get to a trailhead and then switch to Topo to see the terrain and trails for a hike."

Garmin City Select vs. City Navigator

This is a common question -- which should I buy for my Garmin GPS, City Select (CS) or City Navigator (CN)? First of all, go to Garmin's website and check to be sure that your unit is compatible and capable of auto-routing. Otherwise you'll be paying for features that you can't utilize.

Here's the lowdown on the differences. CN was designed specifically for certain StreetPilot units; those units can take advantage of some sophisticated voice-prompted routing information in CN. Though CN can be ran on many handhelds, the map segments are much larger, with several exceeding the 24 MB limit of a number of units.  This is not just a problem in those areas; it makes it difficult to construct a trip along a linear route (can you say road trip?). To further complicate matters, Garmin is discontinuing CS, which has smaller map segments, replacing it with CN. This has caused quite a stir, and Garmin handheld owners aren't happy about it. Apparently the company is listening though, and they seem determined to shrink the map segments in future versions of CN.

CS is getting harder to find as a result of this phase-out. If you can only find CS Version 6, don't worry. As long as it is purchased (and unlocked) after June 1, 2005, you are entitled to a free upgrade. And there is some good news in all this; Garmin has cut upgrade pricing for CN from $150 to $75 and on CS from $75 to $50. My advice? If you've got a handheld, buy CS.

Tracks and Routes

When I was on the road doing GPS mapping demos, pitching my book, it became clear what things confused people over and over again. So today we're going to take a look at one of those and inaugurate a new category -- FAQs (I'm going to dig through the archives and add at least one other post to the category).

The issue today is routes vs. tracks. What is the difference? A route has the following characteristics:

• It is used in the field to guide you from waypoint to waypoint.
• It uses straight-line, as the crow flies, navigation. For highway navigation, the term "route" has a slightly different meaning.
• It is about where you are going.

A track, on the other hand, has the following defining characteristics:

• Tracks are about where you have been.
• They are a record, created by your GPS, which is constantly recording your location. Many people like to use the analogy of an electronic "bread crumb trail," ala Hansel and Gretel.
• They more accurately reflect the shape of the trail or road you have taken than the straight-line segments representing a route.
• They can be downloaded to your computer to create a record of your trip.

We can see the route in the image above, represented in pink, showing straight-line segments between waypoints. The track, shown in yellow, more closely resembles the shape of the trail.

Now many new GPS users don't like this. They want their receiver to guide them at every turn of the trail. But think about it--you don't need your GPS to tell you there is a bend ahead in trail. You just need it to guide you to critical waypoints--trail junctions, campsites, etc.

There is much more to say about both routes and tracks, but I hope that this will give you a basic understanding of the difference.

What kind of maps can I put on my GPS (and what do you mean by raster and vector)?

I received a PM from the Adirondack Forum today that said:

"...I just bought the Garmin 60CS. I have a question about the maps. Do I need the Garmin Mapsource United States Topo or... the one from National Geographics Topo that I think is better for hiking? If I don't buy the Garmin Mapsource Topo, can I download maps in my GPS?..."

This question comes up so often that I felt obligated to post an answer here. The answer is no, you cannot load National Geographic TOPO! maps onto any GPS. You can however, create waypoints and routes with TOPO!, and transfer these to your GPS, but not the maps. Let's look at why not, and what kind of maps you can load onto your GPS.

The National Geographic TOPO! maps look just like a paper map from USGS (example above, at right). That's because they are scanned images, also known as raster imagery. Since they are scans, they have a pretty big file size. That's why it takes 10 CDs to cover the state of California. And that's one of the main reasons that GPS manufacturers have taken a different approach.