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GPS, GIS, and Cartography
I [used to] do alot with GPS, GIS and cartography. I've always loved maps, and orienteering, and this was a technological extension of those passions. TODO: post maps.
GPS
GPS stands for Global Positioning System. GPS currently consists of 24 satellites, each with a small atomic powered clock in them. There are three "steps" in the GPS system.
- These satellites constantly beam the current time down to Earth
- GPS receivers can read this information from several satellites simultaneously
- The GPS receiver can calculate its position based on the differences in time as reported by many different satellites.
It takes 3 satellites to allow a GPS receiver to ascertain its horizontal location, and 6 satellites for a receiver to calculate vertical location. The more satellites that the receiver can read from (and has available to read from), the more accurate the position reporting. In other words a 12 channel GPS that is currently reading 6 satellites is less accurate than an 8 channel GPS that is reading 8 satellites, but a 12 channel GPS has the potential of being more accurate.
GIS
GIS stands for Geographic Information Systems. GIS is, very simplistically, the high-tech version of surveying, cartography, and geologic mapping all rolled into one.
Cartography
Cartography is the practice of creating (or charting) maps.
Accuracy
Since the government has stopped Selective Availability, a 12 channel GPS like mine has an accuracy of 15feet - 40feet horizontally and +/- 50feet vertically. In practice, accuracy of 3feet-10feet is possible under good conditions with a strong satellite availability.
My GPS Receiver
My current GPS receiver (GPS for short) is a Garmin GPS III Plus (depicted to the right). The GPS III Plus is one of the best civillian personal GPS systems available. It can track up to 12 satellites simultaneously, is very durable and waterproof and has a powerful set of features. It doesn't have a lot of fluff of the modern receivers, nor is it in color. Its internal battery can also last an impressively long time.
The GPS III Plus can also be connected to a desktop or laptop to allow live tracking via PC mapping software. I even cobbed together a solution that allows me to interface with my GPS via my Palm handheld (figure 2 to the right). The standard antenna can be replaced with a mounted version that uses a standard BNC connector.
Palm to GPS Adapter (Serial)
The Palm V cradle is a pretty cool little toy. I never use it because I have a Palm V keyboard that also recharges/HotSync's my Palm V. I've always wanted one of those nifty "travel" HotSync cables, but never wanted to shell out the $18 + shipping to buy one -- 3 screws later, I had one. It looks just like the picture to the right, and is surprisingly well-built/durable. A little electrical tape to ward of the static demons, and it'll be a super-duper compact Palm <-> Serial cable.
My Garmin GPSIII+ also has a serial cable, both are DB9 female. After doing some electrical testing, I found that the pinouts used by the GPS were 2,3 and 5. Specifically 2 maps to 3, 3 maps to 2, and 5 maps to 5. Using a pair of DB9 male connectors and some wire, I built a bridging adapter to connect the Palm V HotSync cable I just lifted, and my Garmin PC transfer cable. A little more electrical tape and she was solid as a rock.
That's all there is, really. If you want a really great topology modeler that supports USGS data, I recommend Kashmir 3D- It's freeware, and way cool once you get used to it. It supports GPS syncronization as well (Garmin only) as well as real-time rendering of your surroundings. As for Palm-side software, I use GPilotS. It is a very versatile "cardware" program (the author requests a postcard from users) and has more options than any other Palm-side GPS database I've seen. It also will "act" like a GPS when it is on and in the cradle, so you can use desktop-side GPS apps to interact with your Palm running GPilotS to download/upload waypoints, routes, tracks, etc.