Satellite Imagery Accuracy: What to Expect
Spherical math is exact. Imagery isn't. Here's what really limits the accuracy of any measurement made on a satellite map — and how to think about realistic error bars.
Pixel resolution
Each pixel of consumer satellite imagery represents a real-world distance. In dense populated areas (most of the U.S., Europe, urban Asia), Google's imagery typically resolves down to about 0.3 to 0.6 meters per pixel — clear enough to see lane markings on a road and individual cars in a parking lot.
In rural areas, away from population centers, resolution drops to 1–2 meters per pixel. Boundaries of fields, fences, and treelines remain visible but with less detail. In sparsely-populated areas, expect 5–15 meter resolution. At those scales, a 100-foot property line can be hard to place exactly.
The practical effect: at 0.5 m/pixel, your click can be off by half a meter. At 5 m/pixel, it can be off by 5 meters. For a 1-acre lot (about 200 feet on a side), that's a difference between 1% error and 10% error.
Image age
Satellite imagery isn't live. Google Maps imagery is typically 6 months to 3 years old in populated areas, sometimes much older in rural regions. The age matters when:
- A property has been recently developed (the image might predate the house)
- A field has been recently planted or harvested (different boundaries appear)
- Trees have grown enough to obscure formerly visible boundaries
- Coastlines, riverbanks, or eroded boundaries have shifted
Google sometimes shows the imagery date in the corner; you can also flip between historical images in Google Earth Pro for free if you want to see how a property has changed.
Parallax and tall objects
Satellite imagery isn't a perfect top-down view. Most images are taken at slight angles, and the post-processing — called orthorectification — corrects for this on flat ground but leaves errors on tall objects.
The visible effect: tall buildings appear to "lean" toward the edge of the image. A 40-story building photographed from the south will have its rooftop displaced several meters north of its true position. For roof area measurement of skyscrapers, this matters. For ground-level property boundaries, it doesn't — the ground is usually well-corrected.
For residential roofs (1–3 stories), parallax adds at most a few feet of displacement — negligible for solar sizing.
Cloud cover and shadows
Map providers stitch together cloud-free imagery from multiple passes, but shadows persist. A long building shadow at low sun angle can hide adjacent property features. If you're measuring something the shadow obscures, switch to a different time of year — Google often offers the image taken at a different season.
What this all means in practice
Realistic error bars on satellite measurement, for the most common cases:
- Suburban lot in a populated area: ±1–2% (roughly ±100 sq ft on a 0.25-acre lot)
- Rural acreage with visible boundaries: ±2–5%
- Wooded property with treeline boundaries: ±5–15%
- Building footprints (typical residential): ±2–5%
- Crop fields with strong contrast: ±0.5–2%
- Pivot irrigation circles: ±0.5% (geometry helps)
- Coastline-bounded properties: highly variable; the coastline itself is fuzzy
How to reduce error
- Zoom in. Always click at the highest practical zoom for each corner.
- Click many points on curves. Three points will undermeasure a curved boundary.
- Cross-check. Compare against county GIS, listing data, or a known dimension (a regulation soccer field, a known building footprint).
- Use the latest imagery. If a feature looks faint, check Google Earth Pro for a clearer historical image.
- Avoid measuring through tree canopy. Use treeline edges, not interior canopy.
When the imagery isn't good enough
If the property's boundaries aren't visible — heavily wooded, snow-covered, deep shadow, or rural enough that imagery is coarse — satellite measurement will be a rough estimate at best. Options:
- Check the county GIS portal — they often overlay parcel boundaries on aerial imagery.
- Use a mobile mapping app and walk the boundary with your phone.
- For a definitive answer: hire a licensed surveyor.
Read the survey tools comparison for help picking the next step up.