An orthophoto, orthophotograph or orthoimage is an aerial photograph or satellite imagery geometrically corrected (“orthorectified”) such that the scale is uniform: the photo or image has follows a given map projection. Unlike an uncorrected aerial photograph, an orthophoto can be used to measure true distances, because it is an accurate representation of the Earth’s surface, having been adjusted for topographic relief, lens distortion, and camera tilt.
Orthophotographs are commonly used in geographic information systems (GIS) as a “map accurate” background image. An orthorectified image differs from “rubber sheeted” rectifications as the latter may accurately locate a number of points on each image but “stretch” the area between so scale may not be uniform across the image. A digital elevation model (DEM) is required to create an accurate orthophoto as distortions in the image due to the varying distance between the camera/sensor and different points on the ground need to be corrected. An orthoimage and a “rubber sheeted” image can both be said to have been “georeferenced” however the overall accuracy of the rectification varies. Software can display the orthophoto and allow an operator to digitize or place linework, text annotations or geographic symbols (such as hospitals, schools, and fire stations). Some software can process the orthophoto and produce the linework automatically.
Production of orthophotos was historically achieved using mechanical devices.
An orthophotomosaic is a raster image made by merging orthophotos — aerial or satellite photographs which have been transformed to correct for perspective so that they appear to have been taken from vertically above at an infinite distance. Google Earth images are of this type.
The document (digital or paper) representing an orthophotomosaic with additional marginal information like a title, north arrow, scale bar and cartographical information is called an orthophotomap or image map. Often these maps show additional point, line or polygon layers (like a traditional map) on top of the orthophotomosaic. A similar document, mostly used for disaster relief, is called a spatiomap.
While Google Earth is great for a quick reference, it is sometimes not quite enough to get the job done. For changing construction sites, an updated Orthophoto is crucial for site planning and to show progress to investors. CAD overlays on a Orthorectified image, can very easily determine if everything is in the right spot. A minor mistake on the location of some equipment can cost thousands to rectify and slow down a project. Why not just call us out and have us do the work for you. We can take the CAD plans and overlay them for you, then return the image so all you have to do is confirm the positions. Less worry and less work means more time for you to make sure the job gets done. Everyone wins when the job completes on time, every time.
Here is the link to the full size Orthomosaic we just captured. Zoom down to see all the extra detail. —> EasyZoom