What causes "Last coord not same as first" in dgraph-io/dgraph?

Close the ring by appending a copy of the first coordinate as the last coordinate

Last coord not same as first

Error message

Last coord not same as first

What it means

Thrown by the closed-loop validator in convertToGeom when the first ring of a polygon is not closed — the last coordinate does not repeat the first coordinate. GeoJSON requires linear rings to be closed so the loop can be converted to an S2 loop.

Source

Thrown at types/s2.go:138

	}
	return intersects(l1, l2)
}

func convertToGeom(str string) (geom.T, error) {
	// validate would ensure that we have a closed loop for all the polygons. We don't support open
	// loop polygons.
	closed := func(p *geom.Polygon) error {
		coords := p.Coords()
		if len(coords) == 0 {
			return errors.Errorf("Got empty polygon.")
		}
		// Check that first ring is closed.
		c := coords[0]
		l := len(c)
		if c[0][0] == c[l-1][0] && c[0][1] == c[l-1][1] {
			return nil
		}
		return errors.Errorf("Last coord not same as first")
	}

	validate := func(g geom.T) (geom.T, error) {
		switch v := g.(type) {
		case *geom.MultiPolygon:
			for i := range v.NumPolygons() {
				if err := closed(v.Polygon(i)); err != nil {
					return nil, err
				}
			}
		case *geom.Polygon:
			if err := closed(v); err != nil {
				return nil, err
			}
		}
		return g, nil
	}

View on GitHub (pinned to 759e242be6)

Solutions

Close the ring by appending a copy of the first coordinate as the last coordinate
Run a GeoJSON validator on data before ingesting
Programmatically normalize rings (auto-close if c[0] != c[len-1]) prior to calling the API

Example fix

// before
[[[0,0],[0,5],[5,5],[5,0]]]
// after
[[[0,0],[0,5],[5,5],[5,0],[0,0]]]

Defensive patterns

Strategy: validation

Validate before calling

ring := poly.Coordinates[0]
first, last := ring[0], ring[len(ring)-1]
if first[0] != last[0] || first[1] != last[1] {
    return errors.New("linear ring must be closed (first == last)")
}

Try / catch

g, err := types.ConvertToGeom(str)
if err != nil && strings.Contains(err.Error(), "Last coord not same as first") {
    // auto-close the ring and retry, or reject input
}

Prevention

Auto-close rings during ingestion normalization
Run geojsonhint or equivalent linters on client data
Document the closed-ring requirement for API consumers

When it happens

Trigger: Passing a Polygon (or a ring of a MultiPolygon) whose first ring's last point differs from its first, e.g. [[0,0],[0,5],[5,5],[5,0]] without the closing [0,0].

Common situations: Hand-written GeoJSON, geometries exported from tools that emit open rings, or coordinates edited/truncated by upstream code.

Related errors

AI-assisted analysis of dgraph-io/dgraph@759e242be6 (2026-09-01). Data as JSON: /api/errors/aa3be9d6aebfe5a9. Report an issue: GitHub.