GPS Q&A

Q.I have an interest in using GPS for surveying and mapping. Are there any products that allow me to do both?
K.P. Tulsa, Oklahoma

A.Surveying and mapping using GPS have always been two sides of the same fence. Surveyors and mappers have had different goals and different methods for collecting their data, but the technology being used is virtually the same. Mappers have mostly been interested in descriptive information and have used single frequency (L1) GPS receivers to provide meter- to submeter-level accuracy. For surveyors the goal has always been to collect highly accurate positions, requiring much more sophisticated dual frequency (L1/L2) GPS receivers.
    The clear line between these goals is now becoming blurred, as mappers demand more accuracy in the features which they record, and as surveyors desire to sell their services for GIS data collection projects. These common interests have not resulted in a merging of GPS hardware; GPS receivers are still easily distinguished as mapping-grade (meter or submeter level) or survey-grade (centimeter level). However advances in software are allowing mappers and surveyors to achieve these goals in a similar fashion.
    For mapping purposes, such as wetlands delineation, location of manholes, or relocating sample sites, mapping-grade receivers have performed the task adequately. What use is the base of a tree accurate to a centimeter? How do you even define the base down to this level? However submeter accuracy may not be sufficient if the mapper is trying to locate underground cables which are only a few inches wide to prevent future damage by construction crews. Also, mappers who work with elevations are finding that submeter systems are not adequate for contour mapping or for locating the elevation of features. (A point to note is that vertical accuracy is generally half as good as horizontal accuracy thus a submeter system will be sub 2 meters in the vertical direction.)
    Surveyors on the other hand are interested in centimeter precision – property boundaries, staking out roads and building sites, relocating lost construction markers and so on. In all these situations descriptive information has been limited to the essential facts; accurate positioning is the dominant factor. However, surveyors are now finding that the market for their services to build GIS-style databases is growing, and that their field collection software cannot give them the descriptive information about mapped features that their clients require.
    The difference between single and dual frequency GPS receivers does not assist in the merging of survey and mapping goals. Whereas good mapping systems can receive real-time differential corrections from Coast Guard beacons or differential satellites, survey-grade receivers require a base station with precise reference coordinates, and survey-grade GPS positions require calibrating to an existing control network to achieve true centimeter accuracy relative to existing surveys. Users of mapping-grade receivers are generally not familiar with the practice of establishing a reference station accurately in three dimensions, nor may they be aware of, or even care about, known control points in their area of interest. Also, mappers working with real-time kinematic (RTK) survey receivers may feel constrained by the limit of 10 kilometers between base and rover to achieve centimeter-level positions, after being used to roaming huge tracts of land under the umbrella of continent-spanning submeter real-time services.
    Today there are situations where mappers want to achieve the survey accuracy, and surveyors want to collect descriptive information. However one enters this gray zone with caution, as there are fundamental differences in the way in which surveyors and mappers approach their goals.
    The possible pitfall for surveyors who want to collect feature and attribute information is not so much in the hardware, in fact mapping receivers are easier to set up and use than survey receivers, but in their knowledge of spatial databases such as GIS (Geographical Information Systems).
    Compared to the notational style of descriptive information collected by surveyors for CAD packages, a mapping or GIS database has a consistent structure that may be a part of a complex series of tables, each table drawing from information from the others in a relational fashion. For instance, recording a road as 34th St. N. may be incorrect if related information about that road is contained in another table where the road is called North 34th Street. Geographic databases can draw upon data from many tables via a common link, such as the road name, and if that link does not match, then the related data cannot be accessed.
    For mappers the complexity in collecting centimeter-level positions lies not just in accurately setting up a base station on a known location, but also understanding that absolute GPS precision is different to precision relative to existing control. Surveyors have a thorough understanding of how to assess the quality of this control, how to adjust the GPS data into the control network, and, very importantly, how to minimize error propagation throughout a survey. It is recommended that mappers interested in achieving survey-level accuracy employ the services of professional surveyors at least for the first data collection session, so that they learn the location of existing survey control in their area, the skills required to select and accurately set up a survey base station on one of those control points, and optionally, how to tie their GPS positions into the existing control network.
    For the surveyor who wants to record GIS-style features and attributes, it is recommended that they thoroughly understand how the data they collect will be used. If they are just beginning to set up a spatial database, then database design is a critical step that needs to be performed. If a spatial database already exists, then compatibility becomes the issue. Creating pick lists from the database is a common method of making sure that what is recorded in the field matches what is already in the database. Also, the data needs to be structured into a table which matches the existing database so that new data can be merged into it effectively.
    Surveyors and mappers interested in adopting each others technology will be glad to know that packages are available now to meet these needs. When shopping for GPS systems ask if a mapping system also supports survey-grade receivers. Mapping products are available today which will work with both mapping- and survey-grade GPS receivers to collect GPS data for GIS-style databases. However although the technology and software are converging, education in each others disciplines is still required in order to make the best use of these tools.

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