GIS implementation is putting your plan into practice. As you do so, you will revisit the issues and decisions contained in the plan. Some surprises will occur and changes will likely be necessary, but if the planning was done well they should not derail the effort.
Don't over-commit your GIS effort to ambitious early products. Lay out a schedule that can be met, with useful early products that can be created to help maintain and build political support. The schedule should include a period for hardware and software acquisition, set up and testing, acquisition of digital data, a period of training and practice, a parcel base map pilot project to work out data conversion and design details, and a phased data conversion schedule. It should also contain a timetable for the distribution of GIS data viewing capabilities within the various departments involved, and times when non-GIS specialists will receive some instruction.
Successful base map creation, for a county or city, involves the following steps:
Note that, as of the date of this report, Minnesota's High Accuracy Reference Network (HARN) is nearly complete. This network provides approximately 450 geodetic control marks on a nominal 25 km grid throughout the state. When finished, by the end of 1996, the network will provide a framework of reference stations whose absolute accuracies are in the sub-centimeter range. The HARN will provide an excellent geodetic framework for control densification.
Global Positioning System (GPS) survey techniques and technology can provide high quality and cost effective support for geodetic control densification. GPS surveying involves the capture of signals broadcast by a 24 satellite constellation operated by the Department of Defense. By capturing signals from multiple satellites simultaneously, users can precisely determine their positions on the earth. GPS techniques usually involve placement of one receiver over a geodetic control point whose position has previously been determined (like a HARN station), with other receivers collecting data at monument locations whose geodetic locations are unknown. Data from all receivers are then simultaneously processed to determine the unknown station locations.
There are many vendors who specialize in GPS surveying, and costs for both equipment and consulting services continue to decline. Readers should be aware, however, that navigational and recreational GPS equipment is not suitable for control establishment and that survey-grade GPS systems can be expensive, in the $20,000 to $100,000 range depending on system configuration.
The Public Land Survey in Minnesota was mostly completed in the second half of the 19th century, prior to significant European settlement. At that time survey parties monumented (set township, section, quarter section, meander corners, etc.) both township boundaries and individual section boundaries as well. With the passage of time and development of the state, the majority of those original monuments have been lost or obliterated.
Minnesota law charges county government with maintenance and perpetuation of the PLSS. Many counties (those with county surveyors) have remonumented or are in the process of remonumenting the "government corners."
As a part of any parcel-based GIS, the sectional framework component should be considered primary. Parcel-based GIS creation should include recovery, verification and restoration of all PLSS corners in the location where they were originally set. This is a major undertaking. You should establish geodetic coordinates for at least each PLSS section corner in rural areas and each half section mark in developed areas.
Other examples could include obtaining planimetric and utility line data from electric utilities or street center lines with address ranges suitable for address matching from private data vendors. Datasets from sources such as these will likely require some processing to get them into the geodetic control you are using.
While purchasing data can be fast, and often less expensive than recreating it yourself, remember that the data was not created with your objectives in mind. Geodetic control, data sources, and/or data conversion methods may not meet your standards. This will be particularly true for your engineering objectives. Look at the metadata carefully to determine if data from these sources will be useful to you. If there is no metadata, the datasets themselves will not be useful.
Early consultation with the photography/photogrammetry contractor is important to ensure that the unit of government gets the products they really need in the most efficient way. Given the constraints of weather and seasons, planning for this kind of data acquisition may precede the actual flights by as much as a year.
Using a data conversion contractor has the advantages of greater cost control for the unit of government, a delivery deadline that is more easily enforced, no need to build internal capacity that cannot be used fully later, and the advantage of the contractor's experience in other projects of a similar nature.
In either cases, the best method of conversion for the particular project must be chosen.
The choice between these three methods depends on the nature of your most pressing needs, your resources, who is most active in your GIS effort, and how much short term GIS product you need to produce to keep and build political support. A relatively inexpensive parcel base created by digitizing good parcel maps into adequate control can be worthwhile in showing the value of a parcel based GIS for nearly all the non-engineering uses of GIS in a county or city. In many cases, this limited taste can help excite the appetite, build the support, and demonstrate the need for the more expensive COGO created parcel base to follow.
In board digitizing, the range of variation in positional accuracy can be reduced by recalibrating the digitizing tablet for each section of map that lies inside a set of 4 control points. This is like "rubber sheeting on the fly" for small sections of the overall map. It does not eliminate discrepancies between the overall shape of the digital base and paper parcel map, but it does break these errors up and distribute them widely around the resulting digital parcel map. This results in somewhat lower relative accuracy among the parcel lines, and more positional (absolute) accuracy with respect to the control points. In this way, you can minimize the magnitude of positional errors in your digitized parcel lines.
The weaknesses of this conversion method are slow production rates, technician error in following lines, and possible omissions of features. The strengths are the ability of the human technician to swiftly and correctly interpret complex map data and to incorporate corrections to the data while digitizing, and the greater control over the use of control points and calibration of the source map to the digital map.
Scanning and vectorizing will follow map lines faithfully no matter how convoluted they are. The key to success is having very clean and simple hard copy maps. The automatic processes will follow a dimension line, smudge, or a bit of text just as faithfully as they will follow the parcel lines.
If your source data is curvilinear and complex and your hard copy is very clean (for example, soils maps on mylar), scanning and vectorizing will be superior to board digitizing in speed and accuracy. For complex maps consisting mostly of man made lines (straight and regular curves) and text on hard copy that may not be very clean, such as most hard copy parcel maps, board digitizing is superior
This last point needs more explanation. Legal descriptions of land records have been written by a great many individuals of varying abilities over the generations. They are based on surveys using old technology and new, in winter and summer, under differing circumstances. Each one was written individually for a particular land transaction, referencing little more than the nearest monuments, property lines, and physical features. Chances are, until your parcel base map effort, nobody has tried to put all these descriptions together into a single unified whole and resolve all the discrepancies to the sub-foot level. You will find numerous gaps and overlaps in the map you create from the legal descriptions, and you must have a policy to address these discrepancies.
There are basically two ways to proceed:
If you elect to doctor your parcel lines to make them fit together, you achieve a good looking map with compromised accuracy. It is still good enough for nearly all purposes. If you elect to record lines as described, you will have a more complex map which represents the inadequacies of the legal descriptions. Such a map may wake sleeping dogs that have slumbered for generations. NAIS did not gather data on which method is used more often in Minnesota.
Building a parcel map from legal descriptions using COGO techniques produces high positional accuracy, but it is not problem free. How accurate were the original measurements? Were they recorded correctly? How carefully were landmarks and monuments used in the creation of the legal descriptions? Have any of these landmarks (such as stream channels, lake shores, fence lines, etc.) moved? How precisely was the surveying done? Are there errors in the descriptions? One of the assets of this method is that it will not hide its problems in a plausible but inaccurate picture, as board digitizing can.
In the case of data conversion, a small representative portion of the total data to be converted (perhaps 5% or less) can be processed in the pilot project and the process revised in light of pilot project experience. This is valuable to improve communication, technical procedures, cost estimates, and expectations. It will also reveal more about the true quality of the source data. The single largest unknown, and the major cause of cost overruns in data conversion is the inadequacy of source data. In many cases, it is not even realistic to budget for a significant data conversion project before doing a pilot to test methods and sources.
When growing the group of direct users of the GIS, it may be helpful to provide GIS viewing and mapping software to just a few at first, giving them appropriate training and read-only access to the GIS datasets over a computer network. As in data conversion, using a pilot project for this part of the implementation will help determine who will really need and use the GIS, how they can best be trained, and will test the software & computer network, allowing you to work out problems before involving a larger number of easily frustrated individuals.
E-mail comments or questions to IISAC at iisac@state.mn.us.
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