Navigation nightmare on a desert road

A real-life scare points out how the new computerized systems, while useful, can’t steer you out of every jam.

By Ralph Vartabedian, Times Staff Writer
December 21 2005

Interstate 15 was a parking lot on Aug. 14, as Marty Callner was trying to drive back to Los Angeles from Las Vegas, where he had taken his daughter for a show on the Strip.Callner decided to tap into the navigation system of his Mercedes-Benz SL500R, a $100,000 car equipped with all the available luxuries to keep him comfortable and advanced equipment meant to keep him safe.

But what happened that day still reverberates with the Hollywood producer of concerts by the Rolling Stones and Madonna, among others.

You can read the rest of the Times article here, and it’s a good one, well written and covers some good ground. There are a number of issues people are not being made aware of, often because there are people jumping on the GPS/car guidance bandwagon who really don’t understand the technology. Here’s three major items that you need to know to make informed decisions.

First, GPS itself, the satellites and the signals they send down for locations seem like magic. But they aren’t magic, they are only a tool. Under normal operational conditions, a receiver in a vehicle gets a fairly accurate position on the face of the earth from the system. “Fairly accurate” in commercial, publicly available terms means the position should be within 15 meters (49.2125984 feet) of the true GPS latitude and longitude, 95% of the time. The other 5% of the time. your position could be calculated any amount of distance away from where you truly are. In general, the system operates better, but there’s no error and nothing that can be automatically corrected, or complained of to the manufacturer unless your unit routinely has more error. Now nearly 50 feet isn’t bad, but when you are guiding someone with the popular “turn left/turn right now” sort of instructions you may not always get precise enough results … 50 feet is way more than the width of a 3 lane freeway .. 48 feet, in fact, usually holds 4 lanes or more. You’ll notice I used some “weasel words” in the preceding sentences. Normal operations is one. Since GPS location involves receiving very, very weak radio signals from 12,000 plus miles away, when you are in the middle of record-setting thunderstorms you had better accept the fact there are going to be more unpredictable errors. If you are driving in mountains or in the “urban canyons” between tall buildings you can expect more errors, because the receiver in your car can’t “get” all the signals it wants (some satellites are suddenly shadowed by obstructions) and every time the unit has to search for additional signal sources, time will be lost and errors introduced.

Secondly, there are the maps provided themselves. The first rule of mapping and navigation is that “a map is not the territory’. A map is a man-made representation of the earth’s surface. It can’t always represent accurately every 6 foot wide wagon track across the desert. In addition, maps are drawn using what is called a “datum”. This represents a point where the actual map coincides with a known place on earth. There are, without exaggerations, hundreds of different datum used by map makers. Most maps don’t even tell you their source datum and even if they did, how many drivers want to do the spherical trigonometry to ’slide’ one map on top of another? You can see a great example of this if you use the popular Google Earth mapping tool. Go to Greenwich England, home of the “prime Meridian” that launched our whole system of modern mapping and navigation. It;s easy to see where the Prime Meridian (zero degrees longitude) actually is, there’s a lovely monument there maintained by the British Royal Observatory. One little thing .. turn on the lines of latitude/longitude in Google Erath an observe the quite large distance between where the actual meridian is and where Google says it is ..; reason? GPS, satellite photography and various map companies don’t use the same datum or projections.

Lastly in this current day’s assessment of what’s not yet fully cooked in the GPS navigation world are the in-car services themselves. OnStar and various other services, such as Mercedes uses are people with limitations and legal considerations. They don’t have a magic “all seeing” tool that can find you and pull you back from the precipice. They have the ability to see a customer’s car on a map screen _if_ the customer’s car is within cellular phone coverage. And there are errors even greater than the in-car errors introduced by the time the data gets back to them. You can’t very well expect a service technician at a computer thousands of miles away to tell you if you should turn right onto one wagon track or turn left onto a different one. And if they did have the local knowledge which would be the only way to tell which wagon track is better, how do they keep from getting sued if it turns out somebody took out a bridge since they last traveled the route?

These are three of the main things you need to consider before trusting your driving to one of these services. One other important consideration that isn’t made too clear in the story that has nothing to do with GPS. If you have road service coverage via your car insurance or a third-party company, sys AAA, take a few minutes right now and read what the terms of service have to say about travel on unimproved roads? There’s a really big chance that you aren’t covered even one foot off the pavement. If you’ve got your kids with you and are driving a $100k car and you ho astray and God puts on a lightning show, you’re “position in the world” is not going to count for much … the only thing that can keep you out of trouble is to use your brain before you are in the deep kimchi.