Standardization of Projection, Datum and Unit for Data
Exchange by Maryland State Government Agencies

The Database and Resource Development Subcommittee membership recommended that State agencies adopt the Maryland State Plane Coordinate System using the North American Datum 1983 in Units of Meters as a new data exchange standard to be implemented in Fiscal Year 2000. This recommendation was based on the following discussions. The recommended standard was reviewed and approved by vote at the July 1998 MSGIC quarterly meeting.

The Database and Resource Development Subcommittee of MSGIC met on February 12, 1998, to discuss standardizing on a single Datum, Projection and Unit for exchange of data between State government agencies and for distribution of data through the Technology Toolbox™ program. The following members and guests were present at this meeting.

Rose Davis State Highway Administration
Bill Walsek State Highway Administration
Charles Mueller State Highway Administration
Frank Siano Department of the Environment
Tom Nasuta Office of Planning
Bob Dadd Office of Planning
Tim Baker Office of Planning
Mike Lettre Office of Planning
Dick Bean Department of Agriculture
Ken Miller Department of Natural Resources
Bill Burgess Department of Natural Resources
Don Mulcare National Geodetic Survey, NOAA

DATUM - any quantity, or set of such quantities, which may serve as a reference or basis for calculation of other quantities. In framework and GIS discussions, "datum" usually refers to a set of quantities that serve as a reference for the calculation of positions. A horizontal datum is the set of constants specifying the coordinate system to which horizontal coordinates are referred. A vertical datum is a set of constants specifying the coordinate system to which elevations are referred. In a linear referencing system, the datum serves as the basis for locating the linear referencing system in the real world and consists of a connected set of anchor sections that have anchor points at their junctions and termini. (This definition appears in the FGDC publication "Framework, Introduction and Guide," 1997)

Don Mulcare of NGS provided technical information on the relationship of the North American Datum (NAD) of 1927 to NAD 1983. He also provided significant technical information to the group on the continuous effort to improve the accuracy of geodetic control in North America, including the relationship of NAD 1983, NAD 1983 (1986), NAD 1983 (1991) and NAD 1983 (2000). After the discussions were complete, it was determined that our system of geodetic control posed no limitations on our choice of projections or datums, because it significantly exceeds the accuracies required for our mapping scales.

National Map Accuracy Standards (NMAS) were discussed. It was noted that in mapping applications, a tolerance of +/- 33 feet for 1:12,000 scale products and +/- 40 feet for 1:24,000 scale products was allowed. Therefore, the selection of Datum would not be an issue from an accuracy standpoint.

The group discussed their use of Datums and determined that there were no practical concerns over the choice of Datum from an accuracy standpoint. However, the consensus of the group was that, regardless of the Projection chosen, we should adopt NAD 1983 due to its currency. In addition, it was noted that each agency is currently maintaining data in NAD 1927 and NAD 1983. Eliminating the use of NAD 1927 would reduce redundant data storage requirements.

There was no significant discussion regarding the selection of a vertical datum. The members decided that it was not as significant of an issue as selection of a horizontal Datum, Projection and Unit since we were not dealing with 3-D modeling or vector creation at this time. This issue should be addressed by the subcommittee at a later date.

PROJECTION - a systematic method of representing the whole or part of the curved surface of the earth on another, usually flat, surface. The latitude/longitude values of framework data can be converted to any map projection. (This definition appears in the FGDC publication "Framework, Introduction and Guide," 1997)

The subcommittee members decided to review three commonly used projection systems including Universal Transverse Mercator (UTM), Longitude and Latitude (Geographic), and the Maryland State Plane Coordinate System (SP). The SP projection has been commonly used for mapping applications in Maryland. The Maryland State Plane "projection" is actually a coordinate system that is based, by convention that is established by NOAA NGS, on the Lambert Conformal Conic projection using the GRS 1980 Ellipsoid. It is generally accepted that the Maryland State Plane coordinate system is used as a projection.

Use of the UTM projection was easily ruled out due to the dual Zones 17 and 18 that occur in Maryland. It was noted that it was a past favorite projection used by the Federal government and has been recently adopted by the Chesapeake Bay Program in modified form. This projection system was developed for worldwide military artillery applications and has some limitations for use in civilian applications.

Use of the Geographic projection was seriously discussed by the subcommittee members. It was noted that on page 25 of the FGDC's recent publication ("Framework, Introduction and Guide," 1997) that they are recommending the use of the Geographic projection to facilitate the National Spatial Data Infrastructure (NSDI). The reason is that the Geographic projection is the only system that will provide "seamless" coverage of the entire United States. This assumes, however, that all data is produced in a standard tile format such as 7.5' or 3.75' quadrangles. Given that fact that many jurisdictions have significant quantities of data that are in over-lapping tile systems, sharing of data across political boundaries will most likely result in continued requirements to mosaic data.

Further discussions regarding use of the Geographic projection related to problems with the user community when they try to use positions generated in Longitude and Latitude as opposed to the SP projection. Each agency has experienced significant data accuracy problems when geographic coordinates were used. By contrast, the user community seems to understand the SP projection and produce much more reliable results with regard to data collection activities.

The group also discussed past problems with geographic information systems (GIS) that had difficulty dealing with data in the Geographic projection. It was noted that no particular problems have recently been found. However, no one has recently worked in the Geographic projection and thus, we would not know about current problems. While we doubt that any of the major GIS packages have problems working in the Geographic projection, the lack of recent experience led the subcommittee members to be uncomfortable with the Geographic projection.

The SP projection has the advantage of being well understood and widely used by the mapping and surveying communities in Maryland. Most of the data maintained by Maryland state government agencies is in the SP projection. There were no negative comments about the SP projection by the subcommittee members.

By consensus decision, the subcommittee decided to adopt the use of the Maryland State Plane Coordinated System as the recommended standard "projection."

UNIT - every x and y coordinate can be reported from the origin of a projection system in units of Feet or Meters.

Discussion of adopting the standard Unit in feet or meters turned out to be the most problematic issue and the group could not reach an easy consensus on a recommended standard Unit.

Charles Mueller discussed the fact that surveyors in Maryland currently report their results in State Plane NAD 1983 in units of U.S. Survey Feet. The committee members noted that the Annotated Code of Maryland requires the use of meters, but as a practical matter, no one uses meters to report survey results. It was also noted that surveys are an input to the process of mapping, and as with most inputs, the units will vary. The issue is that once map data is created, it will be exchanged in a common unit. The group also discussed the fact that the correct Unit for use with NAD 1983 is meters.

Through continued discussion, the subcommittee members determined that the majority of data produced by state agencies was currently in Units of meters. The most significant data conversion effort will involve the distribution products for the parcel mapping system referred to as MdProperty View. However, this product will be required to undergo significant conversion regardless of the Datum, Projection, or Unit that is chosen for the newly proposed standard.

By consensus decision, the subcommittee decided to adopt Meters as the recommended standard Unit.

IMPACT TO EXISTING TECHNOLOGY TOOLBOX™ CUSTOMERS

A significant portion of the discussions related to customer impacts caused by adopting any new standard Datum, Projection and Unit. It was estimated that approximately 80% of the Technology Toolbox™ customers are using ArcView or other ESRI products. By adopting Maryland State Plane NAD 1983 Meters, our customers will be able to convert data that they have produced and data that they have previously acquired from State agencies. This conversion can be accomplished using the basic package with no requirement that our customers buy additional modules to perform the conversions.

It is acknowledged that conversion may be a significant effort for some customers, particularly those with minimal GIS experience. However, State agencies must "modernize" their data and GIS systems to comply with current best practice.

In order to keep the confusion to a minimum, the subcommittee members recommended that conversion of data take place during Fiscal Year 2000 with the 1999 release of MdProperty View. This will give MSGIC sixteen months to inform the users of the impending changes and the implications for their operations. MSGIC will provide training opportunities to customers to instruct them in appropriate data conversion methods. The members finally speculated that there will be at least two new releases of software prior to use of the newly proposed standard. It is very likely that significant improvements to Datum, Projection and Unit conversion routines will be written into the various software packages.