Geocoding Accuracy: From Rooftop to ZIP Centroid

## Geocoding Precision Hierarchy

Geocoding assigns coordinates to an address, but the precision depends on the input quality and the geocoding engine's data.

## Precision Levels

| Level | Accuracy | Description |
|-------|----------|------------|
| Rooftop | ±10-30 ft | Coordinates placed on building footprint |
| Parcel | ±30-100 ft | Coordinates at property centroid |
| Interpolated | ±100-500 ft | Estimated position along street segment |
| Street | ±500-2,000 ft | Center of street segment |
| ZIP+4 | ±0.1-0.5 mi | Center of ZIP+4 delivery segment |
| ZIP centroid | ±5-15 mi | Geographic center of ZCTA polygon |
| City centroid | ±10-30 mi | Center of city boundaries |

## When Precision Matters

| Use Case | Minimum Precision |
|----------|-------------------|
| Delivery routing | Rooftop or parcel |
| Insurance rating | Parcel or interpolated |
| Flood zone determination | Parcel |
| Drive-time analysis | Interpolated |
| Market analysis | ZIP or ZIP+4 |
| Regional trends | ZIP centroid |

## Geocoding Failures

When an address cannot be geocoded precisely, the engine falls back to lower precision levels. Common causes:

- New construction not yet in the address database
- Rural addresses with no street-level data
- PO Box addresses (no physical location)
- Misspelled or non-standard addresses
- Addresses in unincorporated areas

## Measuring Geocoding Quality

Every geocoding result should include a **match code** indicating the precision level achieved. Best practices:

- Always store the match code alongside coordinates
- Filter analyses by minimum precision level
- Re-geocode addresses that only achieved ZIP centroid
- Validate high-value addresses manually (insurance, legal)

## Provider Comparison

| Provider | Rooftop Rate | Cost |
|----------|-------------|------|
| Google Maps | 85-95% | $5/1K requests |
| HERE | 80-90% | $4.50/1K |
| Smarty | 90-95% | $1-3/1K |
| Census TIGER | 70-80% | Free |
| OpenStreetMap | 60-80% | Free |

Rooftop rates vary significantly by region — urban areas achieve 95%+ while rural areas may drop below 60%.