Have you watched the discovery channel regarding how they monitor volcanoes ? They can detect movement of less than 1mm. These are not consumer grade GPS devices. Instead this is similar stuff to that used by the military to put bunker busting bombs through windows. It requires special licensing for the more accurate stuff that geologists use. Same type of technology as what is used by commercial earth moving and civil engineering people to level or grade sites to precise tolerances. Again one is talking about less than 1/2" accuracy. This stuff is more than we ever need for boundary line disputes. The surveyor who came out to my site had one. They will never land a survey for any commercial job if they didn't have it (at least in cities).
Determining Your GPS Needs (this is not the most accurate, but portable)
Definitions
Accuracy
Signifies the absolute nearness of measured quantities to their true values.
Occupation Time
Time you should remain at a location, logging GPS data. Occupation time can vary from 1 sec. to 1 hr. depending on application, environment and receiver design.
Environment
Surrounding environment such as trees, water, land or natural / manmade canyon.
Control Surveying
A control survey determines the coordinates of selected reference marks in certain geographic region. Control surveys normally produce very accurate coordinates, which use procedures that are time consuming and expensive.
Static Surveying
Static surveying requires simultaneous observations of at least 5 satellites. Data collection time is usually between 45 - 60 minutes, depending on atmospheric conditions and the number of satellites. Static surveying yields baseline components that are precise to better than + .5 mm + 1 ppm. Recommended base line limits are 40 kilometers or 24 miles or less.
FastStatic Surveying
FastStatic surveying requires simultaneous observations of at least 5 satellites. Data collection time is usually between 8 - 15 minutes depending on atmospheric conditions and the number of satellites. FastStatic surveying yields baseline components that are precise to better than + 1 cm + 1 ppm. Recommended base line limits are 20 kilometers or 12 miles or less.
Kinematic Surveying
Kinematic surveying requires simultaneous observations of at least 5 satellites. Data collection time is usually between 5 - 30 seconds, depending on atmospheric conditions and the number of satellites. Kinematic surveying yields baseline components that are precise to better than + 1 cm + 1 ppm. Recommended base line limits are 20 kilometers or 12 miles or less.
Topographic Surveying
Topographic surveys gather the coordinates xyz or xy of all essential points in a region of interest to produce a map representing the Earth or part of it.
Mapping/GIS Data Acquisition
Mapping / GIS Data acquisition is similar to topographic surveying, the emphasis is on efficiency rather than accuracy. Data collection time is usually between 1 - 30 seconds depending on atmospheric conditions and the number of satellites. The most significant difference between topographic surveying and mapping is that both coordinates and feature attribute information are collected at the same time. The data collected and stored on the data collector in what is known as a GIS compatible format. Features are represented using points, lines or area type entities.
DGPS
Determination of relative coordinates between two or more receivers, which are simultaneously tracking the same GPS signals. The purpose of DGPS is to reduce or remove errors due to the satellite clock, satellite position, atmosphere condition and SA when active. Differential GPS can be performed either in a post process manner or in real time using a radio communication link.
Are you sure?
Maybe it's a statewide thing, but from what I've been told, GPS isn't accurate enough. It also has a higher percentage of mistakes that lead to everything else being off. One mistake on the GPS, and nothing is correct.
Eddie