Using LiDAR Data to Update Winona County’s Zoning Ordinance
By Nick Meyers, Winona County Planning Department
Winona County was one of the nine counties in Southeastern Minnesota that benefited from the LiDAR collection in Fall 2008. The accuracy of LiDAR data has allowed Winona County’s GIS Services to provide broad benefit to the county by becoming an important element in a proposed zoning ordinance update. Winona County GIS is coordinated by staff in the planning department so GIS has always had a strong connection with planning activities. But the idea of using improved terrain data to help understand the effects and implications of the proposed ordinance is new and has gone very smoothly.
Winona County’s topography of steep bluffs, valleys and cold water trout streams provides many opportunities to apply the LiDAR data and has been used to:
Classify lands based on percent slope
Identify lands with a high probability for archeological sites
Use viewshed analysis as a tool to mitigate the effects of development on the bluffs
Identify lands that would be impacted with the proposed ordinance
Areas with high probability for archaeological sites must be studied further before any land-disturbing activities are performed.
Land Classification: The proposed ordinance establishes development limits and restrictions on land as slopes increase. Slopes greater than 12% must have a site plan created by a licensed engineer or professional geologist; when slopes fall between 18% and 25%, development will only be allowed with a Conditional Use Permit; when slopes are greater than 25%, all land-disturbing activities are prohibited. Having an accurate DEM has given the county the ability to confidently identify slopes using the Spatial Analyst Extension in ArcGIS.
Identifying Bluffs: Using the ‘Identify Bluffs’ tool, part of the LiDAR and DEM ArcGIS tool set developed by the Minnesota Department of Natural Resources, allowed Winona County to select topographic features that rise or drop at least 25 feet and have slopes greater than 25%. The tool creates a raster from the DEM that can be customized based on four parameters: percent slope, minimum width of the bluff perpendicular to the slope, minimum area needed to be a bluff, and the minimum change in elevation. By refining these parameters it was possible to create a layer that classifies lands as bluffs within the county.
Archeological Site Probability: The Archeological Sites and Burial Grounds section of the proposed ordinance would not have been possible to administer without the 3-meter DEM that was created from the LiDAR data. This section of the proposed ordinance identifies lands with the greatest likelihood of containing archaeological sites and unplatted burials and then mandates a phase 1 archeological study before development can proceed. Using ArcGIS Model Builder, a model was created based on the predictive model produced by the Minnesota State Archaeologist for Winona County. The model identifies upland terraces or bluff tops within 1,000 feet of the terrace or bluff edge, terraces within 500 feet of rivers and streams, and terraces above floodplains. Before Winona County had the LiDAR data, it would not have been possible to run this model and produce an accurate result using our older 10-meter DEM from USGS.
Viewshed Analysis: Viewshed analysis has been included within the proposed ordinance to protect the scenic qualities of the bluffs along the Mississippi River Valley. The ordinance has established a buffer that includes all bluffs located within one mile of the median of U.S. Highway 61. Within this buffer, all lands that fall within 300 feet of the top of the bluff shall require a Conditional Use Permit (CUP) for development and other land disturbing activities. One of the requirements of the CUP process is to use viewshed analysis as an evaluation tool to assist with the decision making process. This will allow decision-makers to know more precisely how visible a proposed structure would be from nearby areas. The DNR’s ‘LAS to Contour 9.3’ tool, available in the DEM tool set, assisted Winona County in this process; it creates a first-return DEM that includes all vegetation and structures providing for a more accurate representation of the lines of sight when the viewshed tool is used in Spatial Analyst.
Impacts on Development: LiDAR data helped to convey to the County Board, Planning Commission and public the impacts that the proposed ordinance would have on development within Winona County. The main difference between the current and proposed ordinance is the limitation of development on slopes greater than 25%, so having an accurate DEM allowed GIS to be used to analyze the difference in developable land between the current and proposed ordinance.
LiDAR data has given the planning department the ability to identify these areas without having to perform site inspections which will translate into faster and more efficient processing in the future and help to save time and money by decreasing the need for site visits. LiDAR has shown its effectiveness firsthand by allowing GIS to become an important tool for the ordinance update and has shown its versatility by also being used to identify potential karst features and create stream gradients. Future uses will include supporting analyses needed to update the Winona County Hazard Mitigation Plan and to conduct other water planning and natural resource projects.