Reference · print me · lessons 0008–0015

Engine Layer

Spatial reference, georeferencing, storage, topology, COGO, parcel fabric, networks, and the full LADM package map — one page.

Spatial reference stack

ellipsoid   size and shape of the earth model      (WGS 84, GRS 80, Krasovsky)
  + datum   how that ellipsoid ties to the earth   ← the layer that shifts you
  + system  geographic (lat/lon) OR projected (x/y metres)
  + EPSG    the published identifier; WKT carries the full definition

reproject within one datum   = exact maths, lossless
cross datums                 = GEOGRAPHIC TRANSFORMATION — empirical, regional, named

CRS rules

Store cadastral: national projected grid
Exchange: EPSG:4326 · Tiles: 3857
Never measure area in degrees
No .prj = incomplete deliverable; ask
Name the transformation in metadata
Test with round-tripped control points
Wrong datum = systematic shift, drawn perfectly
Enforce in the column: geometry(Polygon, 32637)

Georeferencing

Control points needed: 1 shift · 3 affine · 6 second · 10 third order
Use more than the minimum; spread them out
Low RMS = internal consistency, not truth
Prefer the lowest order that fits
Affine params on a raw image → world file
Orthorectified = terrain displacement removed, pixels resampled
50 cm pixels ≠ 50 cm accuracy — ask for both

Where geometry lives

Geodatabase types

Type	Storage	Versioning
File	Folder of files	No
Mobile	One SQLite file	No
Enterprise	Inside a DBMS	Yes

Versioning = system time (who edited when). Legal time is still yours to model.

PostGIS

GIST = R-tree, the default spatial index
SP-GIST = quad-tree partitioning alternative
geometry = planar, projected → cadastral work
geography = spheroidal → long distances
GeoPackage = OGC single-file exchange format
Constraints bind every client, not just one

Topology

primitives:  nodes · edges · faces        shared boundary stored ONCE
             TopoGeometry references them, grouped into layers, with tolerance

cluster tolerance = min distance between vertices treated as coincident
  gap  >  tolerance  →  flagged as an error
  gap  <  tolerance  →  SILENTLY SNAPPED during validation   ← the dangerous part

Rules that matter

Must Not Overlap	Two claims, one ground
Must Not Have Gaps	Slivers between neighbours
Must Be Covered By	Parcel inside its block
Boundary Must Be Covered By	Agrees with survey lines
Contains Point	One label point per parcel

Under general boundaries

Enforce Must Not Overlap strictly — overlap is a dispute
Treat gaps as findings, sized, routed to the field
Tolerance far below the accuracy class
Record every automated vertex move — it changes evidence
Never auto-snap disagreement into false precision

COGO

The five fields

Direction	North azimuth, decimal degrees, 0–360, clockwise from true north. For curves = the chord.
Distance	Straight lines and multi-segment polylines. Never curves.
Radius / Radius2	Curve parameters; Radius2 for spirals.
Arc Length	Along circular arcs and spirals.

Watch for

Dimensions are the legal record; coordinates are computed
Quadrant N 45°30' E ≠ azimuth 045.5 — convert explicitly
DMS vs decimal: 45°30'00" = 45.5
Closure error as a ratio (1:5,000) — run it on every ingested deed
COGO belongs to fixed boundaries; FFP capture has none — store both, label both

Parcel fabric

record        the legal document; every feature is CREATED BY / RETIRED BY one
              → lineage traversable in both directions
parcel type   parallel fabrics: tax · ownership · easement · subdivision
lines         boundaries carrying COGO dimensions
points        SHARED corners — why an adjustment moves a neighbourhood coherently
connection    ties a parcel to control without being a boundary
seeds         mark an enclosed area so polygons build from lines
AdjustmentLines   adjusted + statistical data after a least-squares run

least squares = COGO dimensions + control points → best-fit coordinates
  free network       evaluates internal consistency, no control
  weighted/constrained   ties to control, improves absolute accuracy

No dimensions and no control means nothing to adjust. The fabric earns its cost on deed-based and surveyed data — not on first-pass imagery tracing.

Networks

Elements & models

Edges travelled · junctions connect · turns restrict movements
Network dataset — transportation; connectivity discovered on demand
Geometric / utility network — directed flow; connectivity maintained during edits
Connectivity = geometric coincidence plus connectivity groups and rules
Crossing lines with no junction correctly do not connect
Open stack: pgRouting over an edge table with source/target/cost

Land questions that are networks

Landlocked parcels — reachability to a public road
Easements and rights of way — linear restrictions
Utility service and access, feeding valuation
Routing field teams across blocks

LADM packages

Party            LA_Party · LA_GroupParty · LA_PartyMember
Administrative   LA_BAUnit · LA_RRR (Right|Restriction|Responsibility) · LA_AdministrativeSource
Spatial Unit     LA_SpatialUnit · LA_SpatialUnitGroup · LA_Level
                 LA_LegalSpaceBuildingUnit · LA_LegalSpaceUtilityNetwork
Surveying & Rep. LA_Point · LA_BoundaryFaceString (2D) · LA_BoundaryFace (3D) · LA_SpatialSource
All inherit      VersionedObject   (+ real-world time in Edition II)

LA_Level             coexisting registration types over the same ground
LA_SpatialUnitGroup  nesting: district · block · adjudication area
Observation types    distance · level · angular · image · TPS · point cloud · GNSS · GPR

Parts:  1 Overview (2024) · 2 Land registration (2025) · 3 Marine · 4 Valuation · 5 Spatial plan
        Essential: 1 and 2.

Conformance means your model maps onto LADM without losing meaning — not that your tables are named LA_*. Extending it for national law is expected practice.

Using this in a design review? Ask your teacher to pressure-test it against your actual requirements — a cheat sheet is a starting position, not a specification.