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Why satellite images don't line up (and what to trust instead)
Put a drawing over one satellite map, switch to another, and the house underneath moves. It is not your drawing. Each provider's photo is placed on the earth with its own error, and they disagree with each other.
Why they differ
- Placement accuracy. Each image is stretched onto the ground using a terrain model and a handful of reference points. Esri's own records for one Adelaide suburb give its imagery a positional accuracy of about ±8.5 m. Plane-flown aerials are usually much better, often within a metre or two.
- Lean. Photos are taken at a slight angle, so tall things lean away from the centre of the photo. A roof edge can sit well to one side of the wall below it, and the direction changes from photo to photo.
- Different dates and sources. Google, Esri and Mapbox each use different satellites and flights, captured at different times, so each shift is different.
What to trust
- Points you measured on site. A GPS reading standing on a corner. A phone is good to a few metres if you hold still and average; a GNSS receiver to centimetres.
- The property boundary from the state cadastre, usually within a metre or so (see find your property boundary).
- The photo, nudged to match those.
In practice
In Plotlay: draw the site boundary (Suggest site boundary), then use Line up the imagery to nudge the photo until the fences sit on the boundary lines. Line up at ground level (fence bases, kerbs), not roof edges. Then tie your drawing down with control points: house corners, set by clicking the photo or by standing on them with GPS. Each point shows how far off it is, so you can see when the fit is good.
More from the blog
- How to put a floor plan on a satellite map, to scale
- Measure a house with an iPhone LiDAR scanner
- Find your property boundary online, free (SA, VIC, QLD and TAS)
- Wall thickness in Australian houses: double brick, cavity, veneer and stud
- Drawing a site plan for a council or development application
- Overlay renovation plans on your floor plan and compare layouts