Over the course of two meetings and multiple conversations, the city/county staff outlined the steps and data necessary for the project. The meetings determined which parcels would be considered developable. A list of assumptions was created, and the exclusion of individual parcels was confirmed. Staff from Community Development, County Zoning, and GIS Services and a planner from a private engineering firm were included in these discussions. To identify suitable parcels, the Parcel feature class was the primary layer used. Selected attributes were queried for the analysis, and a map highlighting each developable parcel was generated.
The first step in the project was to spatially extract all parcels that should be excluded from the project based on the urban services boundary (USB) created and provided by the Community Development Department. The USB, based on multiple considerations such as elevation for water pressure, multiple water and wastewater studies, and other various aspects, includes certain parcels within Helena Valley and excludes all parcels within the Helena city limits. The Select by Location tool in ArcMap was used to exclude specific parcels by selecting from Parcels (the original parcel feature class) all features that have a centroid in the USB feature. Next, parcels located within the Helena city limits boundary feature were removed.
The remaining selected parcels were exported to create a new parcel feature class that represented parcels inside the conceptual development boundary and outside the city limits. This layer would serve as the base feature for additional analysisrelating to landownership, current use, location in a floodplain, lot size, proximity to airports, and other factorswithout affecting the original data. The techniques described in the next section were chosen by the GIS Services Department. In many cases, these data creation and analysis goals can be accomplished in a variety of ways.
Local, State, and County-Owned Government Parcels
One of the assumptions was that development would not occur on government-owned property. Consequently, all local, state, and federal government-owned parcels were excluded using the Select by Attributes tool. Parcels were queried based on the Owncode field, a coded field with values that range from 10,000 (private) to 99,999 (unknown). It is useful for cartographic symbology and querying. Selecting parcels with a code greater than 12,000 removed the federal, state, and local government-owned properties from consideration.
Parcels within Subdivisions and County Zoning Areas
Parcels located within existing or prospective subdivisions (final, platted, proposed, or in-review subdivisions) were also removed. This determination required additional research with the County Zoning Department to confirm preliminary plats located within the project's scope. The existing and proposed subdivision layers were used.
Parcels within a Floodplain
Eliminating parcels within certain portions of the floodplain was essential. The floodplain coordinator provided insight as to the conditions of potential development in floodplain areas. Any parcels located within the 100-year floodplain and floodway were removed using the Erase tool located in the Analysis > Overlay toolbox in ArcToolbox. This process erased any portion of the existing parcel feature that was located within the floodplain layer. Any portion of the parcel that was not included in the floodplain was retained. Some parcels did not maintain their original shape but were still included as potential developable land.
Parcels within the Airport Influence Area
The staff also decided to exclude any parcels within the airport influence area. Building types and heights are limited within the airport influence area. The Erase tool in the Overlay toolset in Analysis Tools in ArcToolbox was used to clip the parcels from the airport influence area. Again, GIS Services used the Clip tool to include potential developable land, even when the parcel's original shape was not retained.
Other Areas Excluded
Staff also decided to exclude any parcels less than an acre in size. Additional parcels were eliminated by manually reviewing their attributes. An attribute list containing the parcel data was created, and discrepancies were reviewed and removed by the Community Development director. After confirming that the analysis was complete, the area and acreage fields were recalculated to validate their respective attributes. Some parcels were clipped during the process, so GIS Services needed to recalculate certain fields to represent their updated spatial representation.
Results and Conclusion
The total acreage and parcel estimates were created using the Clip tool in the Analysis Tools in the Extract Toolset of the Analysis Tools to clip the parcels from the USB. The Erase tool in the Overlay Toolset in Analysis Tools in ArcToolbox was then used to erase the parcels within Helena's city limits boundary. Because some parcels were clipped during the process, the shape area field was recalculated. This located 11,839 total acres that were within the USB and outside the Helena city limits. These results were graphically presented to planning staff in both map and graph formats.
The analysis provided a graphic representation of the potential developable parcels around Helena. The results provided important information for community planners. Acreage was determined by summarizing the Acreage field within the Conceptual Parcels layer. The project detemined that 3,636.6 acres could potentially be developed and approximately four single-family lots could be developed per acre for a total of 14,546 lots. For more information, contact Jason Danielson at email@example.com.
Special thanks to Jerry Grebenc, Frank Rives, Paul Spengler, Sharon Haugen, and Eric Spangenberg for their ideas and comments throughout the project.
About the Author
Jason Danielson is the GIS database analyst for the City of Helena/Lewis and Clark County GIS Services Department. He holds a bachelor of science degree in geography from the University of North Dakota.