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Overview

gdal2gores.py transforms a Simple Cylindrical map projected image into an image with n gores, suitable for printing maps on spheres like tennis balls or globes. The script remaps pixels using an interrupted sinusoidal projection pattern.

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Purpose

Given a Simple Cylindrical map projected image, this tool remaps it to an image with multiple gores for placing or printing the map on a spherical surface. Author: Trent Hare, USGS
Credits: Based on C# implementation by Michal Wisniewski (winski software)

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Installation Requirements

pip install gdal numpy

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Command Syntax

gdal2gores.py -ng number_of_gores infile outfile

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Parameters

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-ng
integer
default:"8"
Number of gores to generate in the output file, based on interrupted sinusoidal projection
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-q
flag
Quiet mode - suppresses progress output
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infile
string
required
Input Simple Cylindrical map projected image (any GDAL-supported format)
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outfile
string
required
Output filename (currently defaults to GeoTIFF format)

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Usage Examples

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Basic Gore Generation

Generate a gore map with the default 8 gores: 
gdal2gores.py mars_map.tif mars_gores.tif
If you don’t specify -ng, the script will default to 8 gores and display a warning message.

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Custom Number of Gores

Create a 12-gore pattern for a globe: 
gdal2gores.py -ng 12 moon_map.tif moon_12gores.tif

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Quiet Mode

Run without progress output: 
gdal2gores.py -ng 8 -q input_map.tif output_gores.tif

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How It Works

The gore transformation algorithm:
  1. Gore Width Calculation: Divides the input image width by the number of gores
  2. Pixel Remapping: For each gore, pixels are repositioned using cosine transformation: 
    newX = goreCenter + cos((-π/2) + (π × y / height)) × (x - goreWidth/2)
  3. Edge Overlap: Uses rounding to increase overlap at gore edges for better alignment
  4. Sequential Processing: Processes line-by-line to handle large images efficiently

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Technical Details

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Output Characteristics

  • Format: GeoTIFF (currently hardwired)
  • Size: Same dimensions as input image
  • Bands: Preserves all bands from input
  • Data Type: Matches input image data type (Byte, Int16, etc.)
  • Projection: No projection metadata (this is a gore image)

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Processing Notes

The script processes images line-by-line, making it capable of handling very large images without memory issues.
  • Processes each band separately
  • Iterates over lines in reverse order (bottom to top)
  • Each gore is processed independently within each scanline
  • Progress is displayed per band unless quiet mode is enabled

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Practical Applications

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Creating Sphere Maps

  1. Print and Cut: Print the gore image and cut along gore boundaries
  2. Apply to Sphere: Align and glue each gore section onto a spherical surface
  3. Tennis Ball Maps: Perfect for DIY planetary globes on tennis balls

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Globe Manufacturing

The gore pattern follows standard globe manufacturing techniques where flat map segments are arranged to minimize distortion when applied to spherical surfaces.

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Example Workflow

# Start with a Simple Cylindrical projection
gdal_translate -of GTiff -a_srs EPSG:4326 planet_map.tif planet_equi.tif

# Generate 8 gores
gdal2gores.py -ng 8 planet_equi.tif planet_gores.tif

# Convert to PNG for printing
gdal_translate -of PNG planet_gores.tif planet_gores_print.png

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Source Code Reference

The core transformation logic is located in:
  • gdal2gores.py:131-144 - Gore iteration and pixel remapping
  • gdal2gores.py:138-139 - Cosine transformation for pixel positioning

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Limitations

  • Output format is currently hardwired to GeoTIFF
  • Input must be in Simple Cylindrical projection
  • No projection metadata is written to output file

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See Also