truncate

truncateGeoJson

Signature

truncateGeoJson(geoJson, options = {})

Description

Reduces the precision of every coordinate in a GeoJSON object to the specified number of decimal places and removes duplicate consecutive vertices from polygon rings.

Rounding coordinates can bring two distinct vertices onto the same point. On polygon rings this produces duplicate consecutive positions, which invalidate a geometry for MongoDB's 2dsphere index (S2). To prevent this, truncateGeoJson deduplicates consecutive positions on each polygon ring after truncation, and re-closes the ring when the closing point was among those dropped.

Deduplication applies only to polygon rings (Polygon and MultiPolygon). Positions of LineString, MultiLineString and MultiPoint are truncated but never deduplicated, since a line may legitimately revisit the same point and S2 rejects duplicates only on rings.

This operator normalizes precision; it does not repair topology. If deduplication leaves a ring with fewer than four positions, the ring is left as is: detecting and repairing a degenerate ring is the responsibility of the validate and fix operators.

WARNING

truncateGeoJson mutates the input GeoJSON object in place and returns the same reference.

Parameters

Name	Type	Required	Description
geoJson	object	yes	Any valid GeoJSON object
options.precision	number	no	Number of decimal digits to keep, in range [0, MAX_COORDINATE_PRECISION] (default: DEFAULT_COORDINATE_PRECISION)
options.consider3D	boolean	no	When true, altitude is taken into account when deduplicating ring vertices (default: false)

Note on consider3D

By default (consider3D: false), two ring vertices sharing the same longitude and latitude are treated as duplicates even if their altitudes differ, and one is dropped. This matches how S2 indexes geometry, using longitude and latitude only. Set consider3D: true to keep vertices that differ solely in altitude. This option is forwarded to isSamePosition.

Returns

Type	Description
object	The same GeoJSON object, mutated

Throws

Throws if geoJson is not a valid GeoJSON object, or if precision is not in range [0, MAX_COORDINATE_PRECISION].

Examples

Truncating a point geometry:

truncateGeoJson({ type: 'Point', coordinates: [10.123456789, 20.987654321] })
// { type: 'Point', coordinates: [10.1234568, 20.9876543] }

Custom precision through options:

truncateGeoJson(geometry, { precision: 3 })

Deduplicating collapsed ring vertices:

const polygon = {
  type: 'Polygon',
  coordinates: [[
    [0, 0],
    [2, 0.0001],
    [2, 0.0002],
    [2, 1],
    [0, 1],
    [0, 0]
  ]]
}
truncateGeoJson(polygon, { precision: 3 })
// The two middle vertices both round to [2, 0]; one is removed and the ring stays closed.

Keeping altitude-distinct vertices:

truncateGeoJson(polygon3D, { consider3D: true })
// Vertices equal in lon/lat but differing in altitude are preserved.