GIS standards setting efforts are currently being conducted by Minnesota Department of Transportation, Minnesota Department of Natural Resources, and the Metropolitan Council. Each of these efforts is focused on the internal needs of the organization. The Governor's Council on Geographic Information is active in setting more general GIS standards for Minnesota in cooperation with these groups and others. The Governor's Council can be contacted through LMIC. The Federal Geographic Data Committee is establishing GIS standards for the federal government. LMIC also has information about this effort.
Maps are flat representations of the shapes, sizes, and locations of features on the earth. The curved surface of the earth cannot be represented on a flat map without distortions of shape, area, scale/distance, or direction. Map projections have been devised to choose, understand, and control this distortion.
The choice of projection determines how features on the map look and what kind of distortion will be present. Different projections are appropriate for representing large, medium, and small areas on the earth's surface. When mapping a relatively small area, like a county or city, less distortion is apparent on the flat map.
At the county and city scale, the most commonly used projections in Minnesota are State Plane, County Coordinate Systems (CCS) based on the State Plane System, and Universal Transverse Mercator (UTM).
State Plane, based on a conic projection, avoids the distortion inherent in displaying the curved earth on flat surfaces by dividing the state into three zones that do not cover much distance north to south. This minimizes the effect of convergence of meridians across the zone, thus allowing the assumption that all the north-south and east-west lines cross at right angles and are scaled the same/are the same length. The unit of measure in State Plane has historically been the foot, however, NAD83 (North American Datum of 1983) State Plane data is published in meters.
UTM, in which both Mn/DOT and LMIC distribute data, is also zonal. It is based on a Mercator projection that is useful for displaying map data that has more of a north-south orientation and covers a larger area than State Plane. Most of Minnesota is contained in UTM Zone 15, with the western-most parts of the state being in Zone 14. In many cases this difference is ignored, and the entire state is treated as if it were in an "extended" zone 15. Maps using the UTM projection, if showing a large enough area East to West, will have noticeable distortion.
Digital maps for GIS all exist in some datum, referenced to some spheroid. The most commonly encountered horizontal datums are North American Datum of 1927 (NAD27), based on the Clarke Spheroid of 1866, and the North American Datum of 1983 (NAD83), based on the GRS spheroid of 1980. For example, most United State Geological Survey (USGS) quadrangle maps are based on NAD27. Mn/DOT currently requires all its mapping to be in NAD83.
Conversion routines do exist for changing datums. The best ones make use of "NADCON" calculations. The best GIS software will enable the user to change projections, coordinate systems, and datums. While these routines do introduce a small amount of error, these errors are not a problem for most GIS applications.
Mn/DOT's standard is UTM meters, NAD83. LMIC provides most data in UTM meters, NAD27, often with a y-shift of -4,700,000. Fortunately, most GIS software can convert data between these common standards.
The State Plane system with measurement units in feet or meters and County Coordinate Systems based on State Plane are the most commonly used in Minnesota local government. They seem to work best for local units of government because, as an orthogonal system, they are easily understood and widely used, and because most land ownership documents are denominated in feet or a compatible English system of measure. We recommend using State Plane, or a County Coordinate System based on it, in the North American Datum of 1983 (NAD83).
This is not the same as having your base map tied to the Public Land Survey system (PLSS) of townships and sections. The PLSS is important in land ownership descriptions, but its section corners may or may not have geodetic control coordinates (coordinates representing a position on the earth in an established coordinate system) established for them.
Geodetic control is critical if you ever use GIS maps from other sources or make yours available for use by others. Having your base map in geodetic control means it fits an established locational standard and other data that fit this standard will overlay correctly with it. Of course this fit is still subject to accuracy limitations inherent in the source of the data and the methods used to create the digital dataset.
If your map layers are not in control, it may be possible to adjust them to more or less line up with control points. However, it takes extra work and always introduces additional error. The magnitude of this additional error is often considerable.
Establishing adequate geodetic control will be an important part of your implementation costs. Many counties will improve and densify their geodetic control before moving forward on base mapping or parcel-mapping efforts. You should consult your civil engineering and survey staff about the density and quality of established control points for a GIS to meet your objectives. For parcel base map control, we recommend establishing real world coordinates at least for each Public Land Survey section corner in rural areas and for each quarter section point in developed areas.
You should establish written standards pertaining to geodetic control in your GIS design. These standards will include projection, coordinate system, datum, required accuracies, and control grid. The quality and density of the grid that is formed by your control point network is another standard you must set. This will depend upon the level of positional accuracy you need to have in your base map. You should consult your county surveyor to help set this standard. Keep in mind that remonumentation and control densification can be expensive.
A schematic map of a location drawn on the back of a napkin will not have correct and consistent scaling or exact compass bearings. The relative distances between features shown will not be correct and the absolute position on the earth for the features shown will not be known. Yet this map can lead you to your destination if its features are shown in the correct relationships. That is, road intersections are encountered in the order shown, a lake is in fact more or less north of a certain road which runs more or less east-west, the bank does adjoin the post office on the side opposite a certain road intersection, etc. These patterns, largely disregarding scale and orientation, are what is meant by topology. The first requirement of a map, sometimes the only requirement, is that it be topologically correct, adequately representing the pattern of things on the earth.
The greater the quality of geodetic control and the positional accuracy of the features for the maps used the greater the relative accuracy is likely to be.
We recommend you set absolute and relative accuracy standards that truly serve your needs, but do not exceed them. For parcel base maps this is often +/- 10 feet for regional planning purposes, +/- 5 feet for local planning purposes, and +/- .5 feet for engineering purposes. The cost of creating the parcel base will increase rapidly as the accuracy standards are tightened.
Single precision can store a coordinate of seven digits without rounding. Thus, a point located at X=1,234,565 feet and Y=4,701,114 feet would remain precise through many processing operations down to the nearest foot. However, if any of the coordinate pairs were more than 7 digits long, rounding would occur and 1 foot of precision would be lost. Thus 44,701,114 would be rounded to 44,701,110. In this example, location in the GIS would be recorded precisely to the nearest 10 feet.
Double precision can handle 14 digits in a coordinate without rounding. This will prevent rounding error on very large coordinate pairs.
Some GIS software can create and use databases in both single and double precision. Some software can handle only one or the other. Double precision may be more expensive to implement and maintain because of higher software costs and greater demands on data processing and storage capacity.
While this is a controversial point, single precision may be good enough for many local governments. High levels of precision can be maintained in a single precision system by using a coordinate shift. For example, LMIC usually performs a shift of Minnesota data in UTM meters by subtracting 4,700,000 from all y-coordinates. Although this may require additional steps when exchanging data, it can make a big difference in software and hardware cost. You should investigate the issue, with your particular needs in mind, by consulting your civil engineer or surveyor, as well as GIS vendors and other users.
Simple aerial photography contains distortion of shape, scale, and area due to several factors: curvature of the earth, differences in elevation on the ground, tilt of the aircraft, and parallax of the camera lens. Photogrammetrists can rectify the photo images to correct some or all of these distortions.
While very useful as data sources, aerial photography and photogrammetry (the correction of distortions inherent in aerial photography) can be expensive. Discuss your objectives with vendors in this field to streamline and rationalize your planned expenditures for photographic products.
For instance, the Minnesota Department of Transportation has available on CD-ROM a statewide GIS dataset of highway centerlines, water features, and political boundaries which is available to other governmental organizations. LMIC has federal data including the National Wetlands Inventory in a GIS format. LMIC also has Digital Elevation Models and Digital Orthophotography available for much of the state. LMIC and the Minnesota Geological Survey have county well index, soils, and various geologic datasets for many parts of the state. These are only a few of the datasets that may be useful in the development of your GIS.
Contact LMIC to learn more about a wide range of Minnesota digital data for GIS.
E-mail comments or questions to IISAC at firstname.lastname@example.org.