Abstract
In recent year, we can easily measure three dimensional location at a certain point using a GPS (Global Positioning System) instrument. Generally we can use the some way for positioning, point positioning, DGPS (Differential GPS) and GPS surveyer. It is said that accuracy by point positioning is about 100 meters, DGPS is 3 meters and GPS surveyer is 1 cm. DGPS and GPS surveyer get high accuracy by means of two GPS instruments. One of them are set on known point and the other, observed point. Both of them must be observed at same time. But, when we use a GPS instrument in undeveloped region, it is difficult to determine known point and observe at same time. Therefore, it is important to examine real accuracy of point positioning. We inspected accuracy of point positioning using GPS instruments produced in some companies.
