In the first 3 posts, I talked about GPS Surveys, Recreational Grade units and Mapping Grade units. In these posts, I talked about the different types of GPS surveying, and how type can be used, I talked about the different types of units, their accuracy, price, features, ease of use and the manufacturers and dealers associated with these units. In this post, I will be talking about GPS units that are used in the field by professional surveyors and engineers to obtain centimeter level accuracy.
A survey grade GPS unit is capable of obtaining centimeter level accuracy in the field when used by a competent qualified individual. A survey grade unit can be set up in different ways; a single rover for static, a single rover for RTK connected to a network, or a rover and base working together. A single rover for static is set up on a point for a specified amount of time and allowed to collect data and then later downloaded to a computer and post processed for the desired accuracy (see part 1 of GPS surveys). A single rover through a cellular modem can connect to a network that transmits correctional data allowing RTK collection (see part 1 of GPS surveys) which has real-time correction of data. The third option has a base and rover which allows the user to set the base on a point of known or unknown coordinates to begin a survey. The two can communicate through radio or cellular modem. After the base has calculated its position, it sends corrective information to the rover allowing RTK data collection.
A single rover with a data collector will start around $12,000 and will cost you a monthly cellular bill for the modem, and if you use a private network, it will probably cost you around $400 per month for access. If you go the base and rover route, a unit will generally start around $20,000 but most people pay a good bit more.
Because these units are used to obtain a high degree of accuracy, there are many things that keep them from being able to obtain a “lock” on a point. These units are affected by high mountains, buildings, tree canopy, radio signals, solar flares and other things that will bend, reflect and jam the GPS signal. These things will keep the units from making an accurate enough determination of their location that they will not give you a solution to the point if it is not within the user specified parameters. This sometimes makes it very difficult for a surveyor to obtain the data he needs when there are physical or environmental factors affecting his unit.
Some of the brands of survey grade units are Topcon, Nikon, Javad, Leica, Trimble, Spectra and Ashtech. You can look these units up online at any land survey equipment dealer and see what they look like and what all goes with them. Along with the units themselves, we sometimes need to buy brackets, cables, batteries, antennas, tripods, rods, cases, computers, software and other accessories to keep these units working the way they are designed to. If you have time, look up some of these things on a dealer website and see what all surveyors need to do their surveys, I think you will find it interesting. Till next keep your lines straight!
McCreless Land Surveying 