Can a GPS be Wrong or calibrated?

Life Technology
1

I have an Eagle Explorer GPS (12 Channel, original model I think) given to me by a friend long ago.

As a tool for scavenger hunts it works fine. My problem is when I tried comparing the longitude and latitude of my GPS to my map (a Pingtung county map you can pick up in a stationary store) or the Map Quest map on line, it seems to put me in the wrong neighborhood. Right town, but a 1/4 mile away.

I don’t know how accurate my map is as we are gettig ready for Chinese New year now.

Can someone tell me how I could calibrate this unit or at least give me the coordinates for the Costco Parking Lot in Kaohsiung or Carrefourre in Pingtung? I’d like to participate in these Geocache games and would not want to frustrate anyone.

2

Install Google Earth, find your destination, and you can read the coordinates from there.

You can get PDAs with embedded 20-channel GPS these days. More precise, and a lot faster to get your fix.

3

I could be wrong but your GPS handheld sends a signal to a GPS satellite. From there the calculations are done to determine your position on earth. After that is complete the satellite will send to your handheld the data and the handheld will interpret it for you as coordinates. In short, the error is not in your handheld. But try wiki for a start for a better explanation: wiki GPS

4

Thanks for your help. The way I understand it is that the GPS is a reciever only. It recieves the data from the various satalites and calculates the location by different times it takes the data to reach the reciever.

Anyway, I don’t think I can justify the budget for a new GPS right now. I’ll play with Google Earth but if anyone has the coordinates like the roof of the parking garage at Costco, Kaohsiung or Carrefour in Pingdong city, please post them as those are the most likely places I’ll be this break.

5

GPS handhelds don’t send out any signals, they are receivers only.

6

You might have set your GPS receiver to the wrong geodetic system. Because the earth isn’t a perfect sphere (it’s got ‘love handles’ around the equator and has other small deformities) you’ve got to use a model to describe the shape. The standard system (as defined by the US) is called WGS84, while Taiwan has it’s own call Hu-Tzu-Shan which is optimized to be accurate around Taiwan.

If your map is on one system, and your GPS receiver is on another then it can produce a difference of up to about 1km. Assuming you can, play with the options and change the geodetic system (there are something like 100 systems available in a normal GPS receiver, but you probably only want either WGS84 or Hu-Tzu-Shan)