Orthophoto
Definition
An orthophoto is an aerial or satellite image that has been geometrically corrected (orthorectified) to remove perspective and terrain distortions so that it has a uniform scale. Unlike raw imagery, distances and areas measured on an orthophoto correspond to real-world values when paired with the correct projection. Orthophotos require accurate sensor models, ground control points, and digital elevation/terrain models. Mosaicking blends multiple frames with seamline optimization and color balancing. Orthophotos are foundational basemaps for planning because they offer photographic realism with map-like accuracy. However, metadata about date, resolution, and remaining artifacts (e.g., building lean in DSM-based products) must accompany any orthophoto to avoid misuse.
Application
Cities update orthophotos to inspect building permits and code violations. Utilities verify encroachments. Environmental teams delineate wetlands and riparian buffers. Emergency responders assess damage quickly after events. Surveyors and engineers trace features where vector data are missing. Archaeologists discover traces hidden in fields through seasonal orthos.
FAQ
What’s the difference between orthophoto and true orthophoto?
True orthos use DSMs and advanced processing to remove building lean, ensuring vertical features are in their true footprint—vital in dense urban areas.
How often should orthos be refreshed?
Depends on growth and risk. Fast-changing cities may fly annually; rural regions less often. Post-disaster flights provide immediate updates.
Can drones produce mapping-grade orthos?
Yes with proper ground control, overlap, and processing. RTK/PPK improves accuracy; QA with checkpoints ensures trust.
What about radiometric consistency in mosaics?
Use color balancing and seamline placement in homogeneous areas; record any residual seams in metadata for transparency.
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