Understanding Polygon Area Calculations in DAS
This article explains how the area of parcels, land areas, or custom farms are calculated within the DAS platform using Geographic Information Systems (GIS).
Area calculations are determined using geographic measurements of the polygon boundaries that define the land area, parcel, or custom farm. It’s important to note that these polygons may consist of multiple disconnected pieces (disjoint polygons). The sections in the rest of this article will aim to break down the methods that lead up to this calculation and aim to help you understand the process and its limitations and margins of error.
How Polygon Measurements Are Made
The area calculations in DAS are performed using advanced GIS algorithms and tools. Below is an general overview of how the process works:
1. Defining the Polygon
A polygon is a geographic shape consisting of multiple vertices (points) connected by straight-line segments.
Each vertex is represented by geospatial coordinates (latitude and longitude).
Polygons may be continuous (a single piece) or disjoint (composed of separate pieces).
2. GIS Tools for Polygon Calculations
DAS utilizes GIS libraries and frameworks (e.g., GDAL, Shapely, or PostGIS) to calculate polygon areas. These libraries follow these steps:
Reprojection to a Planar Coordinate System:
Since customers require areas displayed in metric units, geographic coordinates (WGS84, degrees), normally used for storing boundary data, converted projected coordinates (NSIDC EASE-Grid 2.0 Global, meters) for accurate area calculations.Boundary Integration:
GIS tools compute the area by integrating the boundary coordinates of the polygon. For disjoint polygons, the areas are calculated separately for each piece and then summed to provide the total area.
3. Handling Disjoint Polygons
For polygons composed of separate pieces, the system treats each piece as an individual sub-polygon.
The sub-polygon areas are calculated individually and aggregated to give the total area of the property.
4. Accounting for Geographic Distortions
GIS tools correct for distortions caused by the Earth’s curvature, ensuring accurate measurements regardless of the property’s size or location.
Data Accuracy and Considerations
DAS area calculations are based on geospatial data provided by vendors, processed according to rigorous accuracy standards. Below are the key considerations for data quality:
Positional Accuracy
Cadastre Data Sources: The data is derived from multiple jurisdictional sources, each using varying collection methodologies.
Ongoing Updates: As jurisdictions improve land boundary accuracy, these updates are incorporated into DAS, ensuring data remains current.
Coordinate Systems
Transformation parameters include shifts, rotations, and scale adjustments to align with the latest standards.
Cadastre Geometry Validity
Polygon geometries are validated to ensure they are free of errors such as self-intersections, spikes, duplicate vertices, or null geometries.
Overlaps in polygons are resolved for records though valid overlapping geometries may persist to reflect jurisdictional intent.
Polygon orientation adheres to global standards such as:
OGC Simple Feature Access Specification v1.2.1
GeoJSON Specification RFC7946
Margin of Error
1–2% Margin of Error: Calculations are subject to a roughly 1–2% margin of error, reflecting data quality from vendors and the geospatial processing involved.
NZ-Specific Data: For New Zealand, both vendor-provided areas and geospatially calculated areas are available, providing a comparative margin of error from both sources.
Conclusion
By leveraging GIS tools, DAS we ensure that area calculations are accurate and reliable, while also addressing the inherent limitations and variances in vendor data. Understanding the factors that influence area calculations, including positional accuracy, geometry validity, and margins of error empowers users to make informed decisions based on the geospatial data provided within DAS.