Mittelpunkte

README

Mittelpunkte

Mittelpunkte (German for "Centers" or "Midpoints") is a Go-based command-line tool designed to calculate various types of geographical "center points" for areas defined in ESRI Shapefiles. Whether you are looking for the centroid, the point furthest from the boundary, or the middle point of an enclosing rectangle or circle, this tool provides a suite of algorithms to find the "middle" of any polygon.

Features

• Multiple Calculation Methods: Supports 10 different algorithms for determining the center of an area.
• ESRI Shapefile Support: Reads standard .shp files (and associated .prj for coordinate systems).
• Coordinate System Support:
  • Automatically detects and converts Swiss CH1903+/LV95 (EPSG:2056) to WGS84.
  • Automatically detects and converts German ETRS89 / UTM Zone 32N (EPSG:25832) to WGS84.
• GeoJSON & GPX Output: Saves both the input areas and the calculated middle points in formats ready for GIS tools or GPS devices.
• Visual Illustration: Generates SVG files for each calculation method, illustrating the algorithm's process (e.g., grid points, rays, or bounding boxes).
• Interactive Exploration: If run without a filter, the tool lists all available areas in the Shapefiles and exits, making it easy to find the correct names for processing.
• Elevation Support: Can fetch real-world elevation data via the OpenTopoData API for 3D center of gravity calculations.

Installation

Ensure you have Go installed on your system.

You can install the middlepoints tool directly using:

go install github.com/EverydayRoadster/Mittelpunkte@latest

Usage

Run the tool by pointing it to a directory containing ESRI Shapefiles.

Step 1: List available areas

Mittelpunkte -input ./data/german_shapes

This will list all areas found in the Shapefiles (e.g., city or district names).

Step 2: Calculate center points

Mittelpunkte -input ./data/german_shapes -output ./output/ -filter "Stuttgart"

Flags

• -input: (Required) Path to the directory containing .shp files.
• -output: (Default: .) Directory where the results will be saved.
• -filter: A comma-separated list of area names or level-prefixed names to include. If omitted, the tool lists available areas and exits.
  • Standard: -filter "Stuttgart" matches any area named "Stuttgart" or any level named "Stuttgart".
  • Explicit Level: -filter "Region::Stuttgart" matches only the area "Stuttgart" that belongs to the "Region" level. This is useful for resolving naming collisions across different administrative layers.
• -resolution: (Default: 30.0) Resolution in meters for grid-based methods. Values smaller than 30.0 are automatically adjusted to 30.0 for accuracy and performance. For elevation-based methods (like ReliefCenterOfGravity and ElevationQuartiles), the resolution may be automatically increased (coarsened) to stay under 16,384 elevation points to limit API queries. Elevation data is cached locally in the cache/ directory to speed up subsequent runs.

Calculation Methods

The tool calculates the following 11 middle points for each selected area:

1. BoundingBoxCenter: The arithmetic mean of the minimum and maximum latitudes and longitudes.
2. IntersectionOfOutermost: The intersection point of the lines connecting the extreme North-South and East-West points.
3. CenterOfGravity: The geographic center as proposed by Peter Rogerson (2015); it minimizes the sum of squared great-circle distances by calculating the 3D Cartesian mean.
4. MinimalDistanceSum: The geometric median of the boundary points; the point that minimizes the sum of Euclidean distances to all points on the polygon's border.
5. RotatingBoundingBoxCenter: The average center point of bounding boxes calculated at 1-degree rotation intervals.
6. MinimalDistanceSumEqualSpaced: Similar to MinimalDistanceSum, but uses points sampled at equal 10-meter intervals along the boundary.
7. ReliefCenterOfGravity: A 3D center of gravity that takes the terrain's surface area into account (requires internet access to fetch elevation data, cached locally).
8. ElevationQuartiles: Computes three middle points based on the Q1, Q2 (Median), and Q3 elevation quartiles of the area. It averages the coordinates of all points within 0.5m of each quartile elevation to provide insights into the landscape's distribution.
9. FermatPointF1: The point inside the area that minimizes the sum of distances to all other points within the area.
10. SmallestEnclosingCircle: The center of the smallest circle that completely contains all boundary points of the area.
11. LargestInnerCircle: The center of the largest circle that can be inscribed within the shape (Pole of Inaccessibility).

Output

For each run, the tool creates a sub-directory in the output path named after the input directory. It generates the following files:

• areas.geojson: The boundary polygons of the processed areas.
• areas.gpx: The boundaries saved as GPX tracks.
• middle_points.geojson: All calculated middle points with their method names as properties.
• middle_points.gpx: The middle points saved as GPX waypoints.
• [MethodName].svg: For each method, an SVG file is generated showing the polygon, the calculated midpoint, and a visualization of the specific method's logic.

Coordinate Systems

The tool primarily works with WGS84 (latitude/longitude). However, it has built-in support for:

• Swiss: CH1903+/LV95 (EPSG:2056)
• German: ETRS89 / UTM Zone 32N (EPSG:25832)

If a .prj file is found alongside the .shp file indicating these coordinate systems, the tool will automatically perform the transformation.

License

This project is licensed under the MIT License - see the LICENSE file for details.