gis-domain

name

gis-domain

description

Use this skill to verify GIS correctness of spatial operations, check predicate semantics, validate geometry conventions (winding, closure, DE-9IM), understand edge cases (degenerate geometries, null/empty handling), or answer "does this make sense?" questions about spatial algorithms. This is the domain knowledge oracle for autonomous agents working on vibeSpatial. Trigger on: "is this correct", "should this return", "what does X mean in GIS", "DE-9IM", "winding", "orientation", "edge case", "degenerate", "predicate semantics", "topology", "validity".

GIS Domain Knowledge Oracle — vibeSpatial

Use this reference to verify whether a spatial operation produces the correct result, to understand edge cases, or to validate design decisions.

Question: $ARGUMENTS

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1. Binary Predicate Semantics (DE-9IM)

The 9 standard binary predicates are defined by the Dimensionally Extended 9-Intersection Model (DE-9IM). Each predicate tests a specific relationship between the interior (I), boundary (B), and exterior (E) of two geometries.

Quick Reference

Predicate	Plain English	Key Rule
intersects(a, b)	Any shared points	NOT disjoint
disjoint(a, b)	No shared points	Interior, boundary, exterior all separate
within(a, b)	a entirely inside b	Interior of a inside interior of b; boundary of a may touch boundary of b
contains(a, b)	b entirely inside a	within(b, a)
touches(a, b)	Boundaries touch, interiors don't	Shared points exist but only on boundaries
crosses(a, b)	Geometries cross	Interior intersection has lower dimension than max of inputs
overlaps(a, b)	Partial overlap, same dimension	Interior intersection has same dimension as both inputs
covers(a, b)	No point of b outside a	Like contains but boundary-on-boundary is OK
covered_by(a, b)	No point of a outside b	covers(b, a)
contains_properly(a, b)	b in interior of a	No boundary contact at all

Predicate Edge Cases

Point on polygon boundary:

• within(point, polygon) = True (boundary contact allowed)
• contains(polygon, point) = True
• touches(point, polygon) = True (boundary-only intersection)
• contains_properly(polygon, point) = False (point is on boundary, not interior)
• intersects(point, polygon) = True

Point on polygon vertex: Same as point-on-boundary — vertices are part of the boundary.

Line endpoint touching polygon boundary:

• touches(line, polygon) = True (if only endpoint touches)
• intersects(line, polygon) = True
• crosses(line, polygon) = False (no interior penetration)

Identical geometries:

• within(a, a) = True
• contains(a, a) = True
• overlaps(a, a) = False for points/lines (same dim but identical = not "partial")
• equals(a, a) = True

Empty geometries:

• intersects(empty, anything) = False
• disjoint(empty, anything) = True
• All other predicates with empty input = False

Null geometries:

• Any predicate with null input = None (propagate null)

Predicate Symmetry

Symmetric	Asymmetric
intersects, disjoint, touches, crosses, overlaps	within/contains, covers/covered_by, contains_properly

For asymmetric predicates: within(a, b) == contains(b, a).

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2. Geometry Validity and Conventions

Ring Orientation (Winding)

• Exterior rings: Counter-Clockwise (CCW) = positive signed area
• Interior rings (holes): Clockwise (CW) = negative signed area
• Shoelace formula: signed_area = 0.5 * sum(x[i]*y[i+1] - x[i+1]*y[i])

A positive shoelace result means CCW. This is the OGC/ISO convention used by Shapely, PostGIS, and vibeSpatial.

Ring Closure

Rings MUST be closed: first coordinate == last coordinate. A triangle requires 4 coordinate pairs: [(0,0), (1,0), (0,1), (0,0)].

Minimum valid ring: 4 coordinates (3 unique + closing).

Polygon Validity Rules (OGC)

1. Exterior ring is CCW, holes are CW
2. All rings are closed
3. All rings are simple (no self-intersection)
4. Holes are contained within the exterior ring
5. No two rings cross (holes may touch exterior at a single point)
6. The interior is connected (no cuts that split it)

Invalid Geometry Examples

Invalid Geometry	Description	Result of make_valid()
Bowtie polygon	Self-intersecting exterior ring	Split into two triangles (MultiPolygon)
Touching hole	Hole boundary touches exterior at >1 point	Split into valid parts
Inverted hole	Hole has CCW winding (same as exterior)	Reverse hole orientation
Unclosed ring	First != last coordinate	Close ring by appending first point
Spike/cut	Zero-area protrusion or cut	Remove degenerate parts

LineString Validity

• Minimum 2 points
• is_closed: first point == last point (forms a LinearRing)
• Self-intersection is allowed (unlike rings)

Point Validity

• Always valid unless null or empty
• Empty point: valid geometry with no coordinates

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3. Measurement Semantics

Area

• Polygon: Absolute value of shoelace area (exterior - holes)
• MultiPolygon: Sum of individual polygon areas
• Point/LineString: 0.0 (dimensionally zero)
• Empty geometry: 0.0
• Null geometry: NaN (propagate)
• Units: Square units of the CRS (not geodetic!)

Length

• LineString: Sum of Euclidean segment lengths
• MultiLineString: Sum of part lengths
• Polygon: Perimeter (exterior ring + hole rings)
• Point: 0.0
• Empty: 0.0
• Null: NaN
• Units: Linear units of the CRS

Distance

• Euclidean: Minimum distance between closest points
• Same geometry: 0.0
• Disjoint: Positive value
• Intersecting: 0.0
• Null input: NaN
• Empty input: NaN (or infinity depending on convention)
• dwithin(a, b, d): distance(a, b) <= d

Centroid

• Polygon: Area-weighted centroid (not center of bounding box)
• LineString: Length-weighted midpoint
• Point: The point itself
• MultiPolygon: Area-weighted across parts
• Empty: Empty point
• Null: Null
• Centroid may be OUTSIDE the geometry (e.g., C-shaped polygon)

Bounds

• Returns (minx, miny, maxx, maxy)
• Empty geometry: (NaN, NaN, NaN, NaN)
• Null geometry: (NaN, NaN, NaN, NaN)
• Point: (x, y, x, y) (zero-extent box)

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4. Constructive Operation Semantics

Intersection / Union / Difference / Symmetric Difference

These are set operations on the point sets of two geometries:

Operation	Result	Points in result
intersection(a, b)	A AND B	Points in both a and b
union(a, b)	A OR B	Points in a or b (or both)
difference(a, b)	A AND NOT B	Points in a but not b
symmetric_difference(a, b)	A XOR B	Points in a or b but not both

Return type depends on input and result dimension:

• Polygon intersect Polygon → Polygon, MultiPolygon, LineString, Point, or GeometryCollection
• Line intersect Line → Point, MultiPoint, LineString, or GeometryCollection
• Point intersect anything → Point or empty

Edge cases:

• intersection(a, a) = a (identity)
• union(a, a) = a
• difference(a, a) = empty
• intersection(a, disjoint_b) = empty
• Result may be mixed type (GeometryCollection) for non-trivial overlaps

Buffer

• buffer(geom, distance): All points within distance of geom
• buffer(geom, 0): For polygons, equivalent to make_valid (repair trick)
• buffer(point, r): Circle approximation (polygon with quad_segs segments per quarter)
• buffer(line, r): Corridor around the line
• buffer(polygon, -r): Erode inward (may produce empty if too thin)
• Negative buffer on point/line = empty
• cap_style: round (1), flat (2), square (3)
• join_style: round (1), mitre (2), bevel (3)

Clip (clip_by_rect)

• Fast path for axis-aligned rectangle clipping
• Sutherland-Hodgman algorithm
• Output preserves ring orientation
• Geometry fully inside clip box: returned unchanged
• Geometry fully outside clip box: empty geometry
• Partial overlap: clipped polygon (possibly multi-part)

Dissolve

• Group-by aggregation of geometries
• dissolve(by=column): Union all geometries sharing the same group key
• Interior boundaries between adjacent polygons are removed
• Produces MultiPolygon when unioned parts are disjoint

Make Valid

• Repairs invalid geometries following OGC rules
• Self-intersecting rings → split at intersection points → reassemble
• Inverted holes → reverse winding
• Unclosed rings → close by appending first coordinate
• Spike removal → collapse degenerate parts
• Output is always valid (may change geometry type)

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5. Null and Empty Handling

This is the #1 source of subtle bugs. Null and empty are DIFFERENT.

	Null	Empty
Meaning	"No value" / missing	"Valid geometry with no points"
In pandas	None / NaN	shapely.Point() (empty Point)
Predicate result	None (propagate)	False (except disjoint=True)
Metric result	NaN	0.0
Bounds result	(NaN, NaN, NaN, NaN)	(NaN, NaN, NaN, NaN)
In OwnedGeometryArray	validity[i] = False	validity[i] = True, zero-span offsets
Buffer	None	Empty
Union with valid	None	The valid geometry
Intersection with valid	None	Empty

Propagation Rules

• Metric ops (area, length, distance): Null → NaN, Empty → 0.0
• Predicates: Null → None, Empty → False (except disjoint)
• Constructive: Null → None, Empty → Empty (or context-dependent)
• Aggregation (dissolve): Nulls skipped, empties included

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6. Coordinate System Considerations

vibeSpatial Assumptions

• All kernel computations are planar (Euclidean)
• CRS is stored at GeoSeries/GeoDataFrame level, not per-geometry
• Kernels are CRS-agnostic — they operate on raw (x, y) coordinates
• Distance, area, length are in CRS units (meters for UTM, degrees for WGS84)

When CRS Matters

• Area in degrees (WGS84/EPSG:4326): Meaningless for accurate measurement. Must project to equal-area CRS first. vibeSpatial does NOT do this automatically.
• Distance in degrees: 1 degree != constant meters. For accurate distance, project to local UTM or use geodetic formulas. vibeSpatial uses Euclidean distance only.
• Antimeridian (180/-180 longitude): Geometries crossing the antimeridian may have incorrectly computed bounds, area, and predicates. Split at antimeridian first.
• Polar regions: Euclidean assumptions break down near poles.

Coordinate Precision

• fp64 storage: ~15 significant decimal digits
• fp64 geographic: Sub-millimeter precision at any location on Earth
• fp32 geographic: ~1 meter precision (insufficient for parcel-level work)
• fp32 with centering: Sub-meter precision when coordinates are centered around the compute region

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7. Degeneracy Corpus

vibeSpatial maintains a deterministic corpus of degenerate geometries for testing edge cases. When writing or verifying operations, test against these:

File: src/vibespatial/testing/degeneracy.py

Case	What It Tests
shared_vertex_lines	Two lines meeting at endpoint → touches = True
collinear_overlap_lines	Lines on same line with overlap → overlaps = True
donut_window_polygon	Polygon with hole, clipped by window crossing hole
duplicate_vertex_polygon	Repeated adjacent vertices → stable under overlay
bowtie_invalid_polygon	Self-intersecting → requires make_valid first
touching_hole_invalid_polygon	Hole touching shell → invalid
null_and_empty_polygon_rows	Null/empty rows disappear cleanly

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8. Oracle Testing Contract

When verifying correctness, compare against Shapely (the reference implementation):

# Geometry equality
shapely_result.equals_exact(gpu_result, tolerance=1e-9)

# Float equality
math.isclose(gpu_value, shapely_value, rel_tol=1e-7, abs_tol=1e-9)

# Null handling
# Both null → equal
# One null, one not → NOT equal
# Both empty, same type → equal

Standard tolerances:

• rtol=1e-7 (relative) for fp64 compute
• atol=1e-9 (absolute) for fp64 compute
• For fp32 compute: rtol=1e-3 to 1e-4 depending on kernel class

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9. Robustness Guarantees by Kernel Class

Class	Guarantee	What It Means
COARSE	Bounded error	Result is approximate but within known error bounds
METRIC	Bounded error	Accumulation uses Kahan summation to control drift
PREDICATE	Exact	Correct boolean result even for degenerate inputs
CONSTRUCTIVE	Exact	Output topology is valid; no missed intersections

Exact predicates use adaptive precision:

1. Fast fp64 test with error bound check
2. If |orientation| <= error_bound: mark ambiguous
3. Re-evaluate ambiguous cases with higher precision (expansion arithmetic)
4. Error bound formula: error = (3.0 + 16.0 * eps) * eps * magnitude

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10. "Does This Make Sense?" Quick Checks

Use these rules to sanity-check operation results:

• area(polygon) < 0: Wrong. Area is absolute value. If your kernel returns negative area, you have a winding bug.
• centroid outside polygon: Valid! C-shaped, U-shaped, and ring-shaped polygons can have exterior centroids.
• buffer(polygon, 0) != polygon: Valid if polygon was invalid. buffer(0) is a repair operation.
• intersection(a, b) larger than a or b: Wrong. Intersection is always <= both inputs.
• union(a, b) smaller than a or b: Wrong. Union is always >= both inputs.
• within(a, b) AND within(b, a): Means a equals b (geometrically).
• distance(a, b) = 0 but NOT intersects(a, b): Wrong. Zero distance implies intersection (or touching).
• touches(a, b) AND intersects(a, b): Always true. Touches implies intersects.
• touches(a, b) AND overlaps(a, b): Wrong. These are mutually exclusive.
• contains(a, b) AND disjoint(a, b): Wrong. Mutually exclusive.
• is_valid(make_valid(g)): Must always be True.
• area(union(a,b)) <= area(a) + area(b): Always true (equality when disjoint).