COGO

Coordinate geometry: the boundary described as measurements rather than coordinates. This is how legal documents actually define land — and the reason a parcel can have a shape and a separate, more authoritative, description of that shape.

Two ways to say where a boundary is

A digitised polygon is a list of coordinates: here, then here, then here. A deed does not say that. It says from the iron pin, N 45°30' E, 42.7 metres; thence S 12°15' E, 30.2 metres… — directions and distances, chained from a starting point. That is metes and bounds, and COGO is how a GIS holds it.

The one idea to keep

The measurements are the legal record; the coordinates are a computed interpretation of them. Digitising the polygon and discarding the bearings and distances throws away the authoritative version and keeps the derived one. When a later survey disagrees with your polygon, the deed's dimensions — not your vertices — are what a court reads.

The COGO attributes

A COGO-enabled line feature class carries five double-precision system fields: Direction, Distance, Radius, Radius2 and Arc Length, each holding a measurement or null depending on the line type.¹

Field	Holds
Direction	Used with straight lines, circular arcs, spiral curves and multi-segment polylines. Always the north-azimuth direction along the line, or the chord from start to end point. Stored in decimal degrees, north azimuth, 0–360°, where 0° is true north and angles increase clockwise.¹
Distance	Used with straight lines and multi-segment polylines such as natural boundaries. Never used with circular arcs or spiral curves.¹
Radius / Radius2	Curve parameters. Radius2 covers the second radius of a spiral curve.
Arc Length	Used with circular arcs and spiral curves — one of the parameters defining their shape, alongside chord distance, radius, central angle and tangent length.¹

Note what Direction means for a curve: the straight chord between its endpoints, not a tangent. A curve therefore needs Direction plus radius and arc length before its shape is determined — three numbers, not two.

Bearings, and why the format bites

Surveyors write directions in several conventions and they are not interchangeable:

North azimuth      045.5000     clockwise from north, 0-360      ← stored form
Quadrant bearing   N 45°30' E   quadrant + angle from N or S    ← deed form
South azimuth      225.5000     clockwise from south             ← some jurisdictions
Degrees-minutes-seconds vs decimal degrees:  45°30'00"  =  45.5

ArcGIS stores north azimuth in decimal degrees and displays whatever format you ask for.¹ The trap is at the boundary of your system: a data-entry clerk typing quadrant bearings into a field expecting azimuth produces parcels that are the right size and shape and rotated into the wrong place. Store one canonical form; convert explicitly at the edges; never guess from the value.

Closure — the free quality check

Chain a parcel's bearings and distances from the start point and you should return exactly to where you began. You never do. The gap is the closure error, usually expressed as a ratio — a misclosure of 1 part in 5,000 means the error is one unit per five thousand travelled.

This is the cheapest data-quality test in land administration, and it works on paper documents before anything is digitised. It catches transcription errors — a transposed digit, a dropped segment, a bearing entered as quadrant when it was azimuth — the moment the dimensions enter the system. It costs one function and it should run on every deed you ingest.

Where FFP complicates this

COGO belongs to the fixed boundary world (lesson 0002): measured, definitive, coordinates computed from dimensions. Most donor-project capture is the opposite — general boundaries traced from imagery, with no bearings to record at all. Both usually exist in the same country. A serious system stores COGO dimensions where they exist, alongside imagery-traced geometry elsewhere, with the accuracy class saying which is which. Forcing one mode on all data destroys information in one direction or manufactures it in the other.

Drill 1 — Which COGO field?

Drill 2 — Recall

Your primary source

Esri, Introduction to COGO — the definitions of the five attributes and how each line type uses them:
pro.arcgis.com — Introduction to COGO

Then About COGO descriptions for how metes-and-bounds text maps onto those fields — useful when you are writing an importer for deed documents.

Ask your teacher. Good next: "how do I model COGO dimensions in PostGIS?", "write a closure check for a traverse", "how do dimensions and imagery-traced geometry coexist in one table?"