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Using a GIS to Locate Land Suitable for Rezoning in the Town of Duxbury, MA
Click HERE for graphic. ABOUT THIS REPORT The Metropolitan Area Planning Council is the designated regional planning agency for 101 cities and towns in metropolitan Boston. The Council helps its member communities plan in the areas of economic development, housing, demographics, transportation, water, solid waste and open space. 1990-1991 MAPC OFFICERS Franklin G. Ching, Ph.D., President, Natick Marjorie A. Davis, Vice President, Wenham Martha K. Gjesteby, Secretary, Cohasset Jay J. Donovan, Treasurer, Wilmington MAPC EXECUTIVE DIRECTOR David C. Soule PARTICIPANTS IN THE PROJECT Town of Duxbury Paul Halkiotis, Town Planner Metropolitan Area Planning Council Geographic Information System Lab Douglas Carnahan, Ph.d., GIS Coordinator Ariane Oberling, Cartographer University of Massachusetts/Boston Digital Graphics Lab John Murphy, Manager of Graduate Computing Richard Gelpke, Assistant Professor of Geography Ray Sawyer, student INTRODUCTION During the spring of 1989, the Metropolitan Area Planning Council (MAPC) determined that it could greatly enhance the scope of its services available to its 101 member communities by the installation of a geographic information system (GIS). Such a system could not only increase our ability to create maps and perform complex geographic dialyses at the regional, subregional, and local levels, but also give us the ability to relate key data files, such as MAPC's Development File, to specific geographical locations. As part of our continuing interest in developing a town-level digital database, MAPC has begun to incorporate our GIS into many of our local mapping projects. One of our long-term goals is to provide open space mapping and zoning information for all of our member communities. Concurrent with our acquisition of the GIS, the University of Massachusetts at Boston was setting up the Digital Graphics Lab (UMB\DGL). They were also looking for a pilot project and decided to collaborate with us on a project for the Town of Duxbury. PILOT PROJECT MAPC solicited projects from its member communities during the fall of 1989. In January of 1990, the proposal submitted by the Town of Duxbury was selected because its scope was most in line with our capabilities at that time. Town Planner Paul Halkiotis and the Town Planning Board had been looking at the opportunity and need to increase the amount of land zoned as Neighborhood Business Districts in the town west of Route 3. We were asked to digitize the parcel, zoning and open space maps of Duxbury for the area west of Route 3, and to examine these and other environmental constraints to identify parcels suitable for rezoning. Raw cartographic data are put into digital format through a process called digitizing. In digitizing, a map is taped to a special table that has a dense mesh of very fine wires in it. Each point where the wires intersect has a specific, known coordinate value. A special device is run over the map that can sense these intersections and transmit their coordinates to a computer. The computer, running Arc/Info or similar software, has the ability to reconstruct lines and polygons from these points and reassemble these into a copy of the original map. One can then use a relational database, such as Dbase or Arc/Info, to link information to the individual lines and polygons that comprise the base map. The generalized scope of our pilot study did not warrant our digitizing directly from the assessors' plans. The town was able to provide us with a mylar composite assessors' map at 1"=1,000' (1:12,000), which was at a large enough scale to meet our needs. In addition, the data needed to produce our primary screens (zoning, including zoned wetlands, open space, and existing open water) were only available at the 1"=1,000' scale. This information had been compiled directly onto mylar copies of the composite assessors' maps. Digitizing from mylar base maps avoids the data distortion that occurs when paper maps shrink and swell with the Non-Profit Conservation Land Trusts - such as the Audubon Society, Duxbury Beach Residents, Inc., Art Complex, Inc., Ballou Channing, Inc., Bay State Company, Boys and Girls Camps, Inc. and Mann Prentiss. Once parcel numbers were assigned to the open space parcels in western Duxbury, the parcel database was queried to identify the town- owned land. Ponds Coverage: Since the shores of many of the larger bodies of fresh water form the boundaries between different zoning districts and also between certain parcels of open space, the ponds were digitized from the Town of Duxbury Open Space and Trail Map. Zoning Map (figure 2): The zoning data came from the most recent Town of Duxbury Zoning Map, which was compiled in 1982. In addition to zoned wetlands, the zoning map provided us with information regarding aquifer protection districts in the town. The zoning districts for the entire town were mapped. Duxbury is divided into the following zoning categories: Residential Compatibility Districts, Planned Development Districts 1- 2-3, Neighborhood Business Districts 1-2-3-4-5, Dunes Protection Districts, Wetlands Protection Districts, and Aquifer Protection Districts. For a more detailed description, see the Town of Duxbury Protective Bylaw, available through the town. MassGIS coverages: Several of MassGIS's coverages were used to perform some preliminary screening. These included their public well sites, large- and medium yield aquifers and streams coverages. For more detailed information about these datalayers, please contact MassGIS at the Executive Office of Environmental Affairs, Boston, 617/727-3888. Hydrography - Only the streams were used from this datalayer. The streams were taken from the USGS 1:100,000 Hydrography Digital Line Graph files, and modified by MassGIS, and projected into state plane coordinates. Public Water Supplies - This datalayer contains the active municipal and other public water supplies, both wells and surface water. It was compiled by the USGS Water Resources Division from the State Water Use Database System. Aquifers - This datalayer shows areas of potential groundwater yield as defined by the USGS in its 1:48,0000 scale hydrologic atlas. High yield aquifers for Duxbury were defined as having a yield of more than 300 gallons per minute and a transfer of greater than 4,000 square feet per day. Medium yield aquifers are defined as having a yield of 100 to 300 gallons per minute and a transfer of 1,400 to 4,000 square feet per day (see figure 3). MacConnell Land Use maps: The town of Duxbury also requested a plot of the most recent (1985) land use map for the Town. This coverage was created by the Resource Mapping Project, Department of Forestry, University of Massachusetts at Amherst and was processed by MassGIS. Twenty one land use categories were interpreted from 1:25,000 scale aerial photographs, including residential, commercial and open lands. For more information contact the Department of Forestry. MAPS FOR ANALYSIS Two analytical maps were produced as a result of the study. These showed water/wetland/well constraints by type and in combination with existing open space. Each had as its base the parcel map* and major local, state and federal roads. In addition, in order to ensure that new businesses could expect high visibility and good access to the road network, we defined an eighth of a mile corridor around these major roads and defined this as our study area boundary. The high and medium yield aquifer coverage from MassGIS was not used in the final versions of the analytical maps. At this time we decided not to remove this land from consideration due of its large areal extent and the fact that some types of dispersed, low-density and environmentally benign commercial activities might be appropriate within this area. Aquifer, Wetland and Water Resources Constraints Map (figure 4) Hydrology, wetlands and public well sites were compiled onto one map. Since Duxbury is largely unsewered, it became important to assess the extent of these features to protect the purity of the water supply. This also included bodies of open water, such as South River Reservoir. Data used for the water/wetland/well constraints map included: zoned wetlands and aquifer protection districts, public well sites, streams, and ponds. Land Suitable for Rezoning Map (figure 5): The final composite map shows all the areas that have development constraints and are unsuitable for commercial rezoning consideration in western Duxbury. The areas not covered by a pattern represent those areas which may be suitable for rezoning as neighborhood business districts, pending further field investigation. Our preliminary environmental screening process shows that these areas do not have development constraints on them and are not areas of protected open space. Data used for final map included the protected open space, zoned wetlands and aquifer protection districts, public well sites, streams and ponds. *Due to the large scale of these maps published here, the parcel boundaries have been omitted from them for the sake of clarity. RESULTS As a result of our researches, on September 5, 1990 Paul Halkiotis, MAPC and UMB\DGL were able to recommend to the Board of Selectmen and the Planning Board six areas in western Duxbury which would be suitable for further study for rezoning as Neighborhood Business Districts. These areas were defined as land adjacent to the following intersections: North & Myrtle Streets, Union & Keene Streets, Congress (route 114) and Keene Streets, Congress and Union Streets, Summer and Keene Streets, and Summer and Birch Streets. These areas have several factors in common. They have good access and visibility along major local roads, they are not in the zone of contribution to any existing or proposed public well site, and large wetlands, ponds, and streams are absent from these areas. As the number of potential parcels is refined, the town will be able to access the parcel database for detailed information on ownership, use, land value, and other fundamental criteria before a final determination as to which sites are most suitable, both economically and environmentally, for rezoning consideration as Neighborhood Business Districts. FURTHER STEPS As new GIS users, this pilot project was very much in keeping with our skills when we began last January. Through the year it provided us with endless opportunities to learn more about Arc/Info and to strengthen our computer mapping skills. At MAPC we are now working to incorporate the Open Space Plan for the Town of Dedham with our GIS. In addition, we have started a more analytical project combining work from a groundwater protection study and a Route 128 Corridor Planning Study for the North Suburban subregion of MAPC. UMB\DGL is planning to research projects in the Buzzards Bay area and to work with the Sudbury Valley Trustees. We look forward to our ensuing individual projects and to further collaboration between UMB\DGL and MAPC. We would welcome discussions on and proposals for additional GIS projects from our member communities, state and regional agencies, and others. Appendix A LEARNING TO WORK WITH ARC/INFO Since most large digitizing projects involve more than one person it is essential to formulate a digitizing strategy before any work is done. Coordination should involve such matters as how the puck will be placed at intersecting arcs, and if arcs will be digitized as single lines or connected lines, to name a few. It is necessary to set up and stick to a strict working schedule. Digitizing is repetitive and mentally exhausting work that is critical to the integrity of the resulting database. A well-paced schedule helps everyone avoid mistakes that become extremely costly in terms of time and money. Obtaining spatially correct, real-world coordinates is essential in the creation of a master coordinate file to which all data layers can be keyed. This helps ensure that all our data layers will line up for both analytic and display purposes. It is also extremely important to carefully select an accurate and up-to-date base map. For example, an entire housing project (over 35 lots) was discovered to have been incorrectly portrayed on the composite assessors' map. Later in the project we were able to acquire a set of the assessors' master sheets, which were checked against the final digitized map. This cross check revealed errors in the composite base map and resulted in the discovery of several new subdivisions created since the composite map had been drawn. This added another three to four weeks of editing to the project. Midway through the project we began to realize that the scale at which data is input into the machine is of critical importance to the maps produced. Though map data can be output at any scale desired, no miracle of technology will make data digitized at 1:25,000 and 1:100,000 seem other than crude when used in conjunction with data developed at 1:12,000. As a result, a lot of time was unexpectedly spent digitizing such key features as local major roads, ponds and the Duxbury coastline before the thematic maps could be automated. Arc/Info is a difficult to learn, very demanding, and very powerful series of programs. Once the databases are created and the software mastered, Arc/Info can be an incredibly powerful tool for analysis and map production. APPENDIX B SOFTWARE AND HARDWARE Arc/Info, the software used in this project, can export its data sets in a wide variety of formats, including DXF (ascii interchange format), IGES (initial graphics exchange standard/uncompressed ascii), ATLAS, DLG (digital line graphs), MOSS (mat) overlay statistical system), and GBF/DIME. Independent utilities are being developed to interchange with MAPINFO, a low-end geographic information system. Arc/Info is available on most of the commonly used platforms, from PCs to Mainframes. At MAPC, work was done using a PC; at UMB/DGL a VAX was used.
