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Split Bounding Boxes that cross the 180th Meridian


The BTAA Geoportal does not display bounding boxes that cross the 180th meridian (also known as the International Date Line.) In these circumstances, the West coordinate will be a positive number, but the East coordinate will be negative.


One way to mitigate this is to create two bounding boxes for the OGM Aardvark Geometry field. The Bounding Box value will be the same, but the Geometry field will have a multipolygon that is made up of two adjacent boxes.

The following script will scan a CSV of the records, identify which cross the 180th Meridian, and insert a multipolygon into a new column.

The script was designed with the assumption that the input CSV will be in the OGM Aardvark format, likely exported from GBL Admin. The CSV file must contain a field for Bounding Box. It may contain a Geometry field with some values that we do not want to overwrite.

This script will create a new field called "Bounding Box (WKT)". Items that crossed the 180th Meridian will have a multipolygon in that field. Items that don't cross will not have a value in that field. Copy and paste only the new values into the Geometry column and reupload the CSV to GBL Admin.

import csv

def split_coordinates(coordinate_str):
    if not coordinate_str:
        return ''

    coordinates = coordinate_str.split(',')

    west, south, east, north = map(float, coordinates)

    if west > 0 and east < 0:
        polygon1 = f'({west} {south}, {179.9} {south}, {179.9} {north}, {west} {north}, {west} {south})'
        polygon2 = f'({-179.9} {south}, {-179.9} {north}, {east} {north}, {east} {south}, {-179.9} {south})'
        return f'MULTIPOLYGON(({polygon1}), ({polygon2}))'

    return coordinate_str

# Specify the input CSV file path
input_file = 'your_input_file.csv'

# Specify the output CSV file path
output_file = 'your_output_file.csv'

# Specify the name of the column with the Bounding Box coordinates
coordinate_column = 'Bounding Box'

# Specify the name of the new column to store the updated coordinates
new_coordinate_column = 'New Bounding Box (WKT)'

# Read the input CSV and process the coordinates
with open(input_file, 'r') as file:
    reader = csv.DictReader(file)
    fieldnames = reader.fieldnames + [new_coordinate_column]

    with open(output_file, 'w', newline='') as output:
        writer = csv.DictWriter(output, fieldnames=fieldnames)

        for row in reader:
            bounding_box = row[coordinate_column]
            new_bounding_box = split_coordinates(bounding_box)
            row[new_coordinate_column] = new_bounding_box

print("CSV processing completed!")