Vector Editing, Attribute Tables, and Field Calculator

Vector Editing, Attribute Tables, and Field Calculator

Why Accurate Vector Editing Matters

Parcel data forms the legal backbone of land records, property tax assessment, and boundary surveys. When polygons are missing, overlap, or leave gaps, downstream analysis produces errors that can carry real legal and financial consequences. Your job as a GIS technician is to ensure that every feature you add or modify is spatially correct, topologically clean, and fully attributed. This lesson covers the core skills: digitizing with snapping, running topology checks, and computing attributes with field calculator expressions in both ArcGIS Pro and QGIS.

Snapping: Connecting Features Precisely

Snapping forces your cursor to lock onto an existing vertex or edge when you draw near it, eliminating the tiny gaps and slivers that are invisible on screen but break topology checks. In ArcGIS Pro, open the Snapping toolbar (Edit tab → Snapping menu) and enable Vertex and Edge snapping. Set the tolerance to 1 map unit in the Snapping Options dialog; at a typical parcel scale this is 1 foot or 1 meter depending on your coordinate system. A yellow snap indicator appears when the cursor locks on.

In QGIS, go to Project → Snapping Options. Set the mode to All Layers or restrict it to your parcel layer. Choose Vertex and Segment as the snap type and enter a tolerance of 1 map unit or an equivalent pixel distance. The magnet icon in the toolbar must be active (highlighted) for snapping to function during an edit session.

Best practice: Before digitizing, zoom in to the area where the missing polygon meets existing parcels. Confirm that your first vertex snaps to an existing corner. Work methodically around the lot, snapping each vertex to existing edges so the new boundary shares geometry with its neighbors.

Topology Rules and Checking

Topology describes the spatial relationships between features. For a parcel dataset, the two critical rules are:

• Must Not Overlap — no two parcels can share area.
• Must Not Have Gaps — no empty space should exist between adjacent parcels (within the study area boundary).

In ArcGIS Pro, add a Topology to your geodatabase (Catalog pane → right-click feature dataset → New Topology). Add both rules, then run Validate Topology. The Error Inspector lists every violation with its location. A passing result shows zero errors.

In QGIS, use Vector → Geometry Tools → Check Validity to catch self-intersections, then install the Topology Checker plugin (Plugins → Manage and Install Plugins). Add the same two rules and click Validate All. Errors appear as point, line, or polygon markers you can click to navigate to.

Data safety: Always work on a copy of the original shapefile or geodatabase feature class. Rename your working file clearly (e.g., `parcels_edit_v1`). This preserves the original in case edits need to be undone.

ArcGIS Pro Field Calculator: Area and Perimeter

Once your polygons are topologically clean, calculate geometry attributes. Open the Attribute Table (right-click the layer → Attribute Table). Add a new field named `AREA_ACRES` (type: Double). Right-click the field header → Calculate Field.

In the Field Calculator, set the language to Python 3 and enter:

```

!shape.geodesicArea@acres!

```

This expression reads the geodesic (curved-earth) area directly from the feature geometry and converts it to acres. For perimeter in feet, add a field `PERIM_FT` and use:

```

!shape.geodesicLength@feet!

```

Geodesic calculations account for the curvature of the Earth and are preferred for accuracy over planar calculations, especially for larger parcels or those near the edges of a projection zone.

After calculating, cross-check your computed acreage against the legal descriptions on the handout. Legal descriptions often state lot area to the nearest hundredth of an acre. Your computed value must fall within 0.01 acres of the stated value to pass this lab.

QGIS Expression Builder: Conditional Flagging

The QGIS Field Calculator (pencil icon in the attribute table toolbar) supports a full expression language. To flag undersized parcels, add a new text field named `SIZE_FLAG` (length 20 characters). In the expression window, enter:

```

CASE

WHEN $area / 43560 < 0.25 THEN 'undersized'

ELSE 'ok'

END

```

Here `$area` returns the area in the layer's native square units. Dividing by 43,560 converts square feet to acres. If your layer is already in square meters, divide by 4,046.86 instead. Always confirm your layer's coordinate reference system (CRS) and units before writing expressions.

Once the field is populated, right-click the layer → Filter (or use Select by Expression) to select only records where `SIZE_FLAG = 'undersized'`. Then right-click → Export → Save Selected Features As to write those parcels to a new shapefile or GeoPackage layer.

Coordinate Reference Systems and Units

Before any area or length calculation, confirm the layer CRS in ArcGIS Pro (Layer Properties → Source) or QGIS (Layer Properties → Information). A parcel layer in a State Plane coordinate system uses feet as its unit; one in UTM uses meters. Using the wrong unit in your expression produces wildly incorrect results. The `geodesicArea@acres` syntax in ArcGIS Pro handles the conversion automatically, but in QGIS you must apply the correct divisor manually.

Workplace Safety Notes

GIS lab work is low-hazard but not hazard-free. Ensure workstations are ergonomically set up: monitor at eye level, wrists neutral, breaks every 45–60 minutes to prevent repetitive-strain injury. Secure loose cables to prevent trip hazards. Never save edits to a shared network layer without coordinating with your team to avoid overwriting others' work. Follow your organization's data governance policy for any parcel data containing personally identifiable property-owner information.