10/7/2015 GEM Lecture 15 Content Photomap -- Mosaics Rectification

10/7/2015 GEM Photomap – –Great savings in time may be achieved by the direct usage of aerial photographs and the tracing of the details and the addition of cartographic details. Such a map is called a photomap. Useful for planners. – –Maps are often urgently required for natural resource inventories and for planning development projects – –Although the introduction of computer-assisted techniques have greatly speeded up the map production process, photomaps can still be seen in use in developing countries

10/7/2015 GEM Mosaics – –The notion of getting a much larger picture of a region by pasting images of individual scenes into a single composite goes back to the early days of aerial photogrammetry – –The individual photos can be laid out side by side, commonly with some overlap, to construct mosaics. The process is a bit like jigsaw puzzles.

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10/7/2015 GEM – –The aircraft is flown such that the aerial camera takes the photographs in sequence, so that there is always about 50% overlap (common area) between each successive picture, normally accomplished by an automated camera shutter at a timing interval controlled by altitude, air speed, and camera properties. – –On the next parallel line the pilot attempts to traverse the ground at a lateral distance that produces up to 40% sidelap. Both end-to- end and side overlaps allow for obtaining a three-dimensional or stereo effect.

10/7/2015 GEM – –A single or number of aerial photographs, each covering portions of the entire area of interest (AOI). These photographs can be mounted together to form one photographic image covering the whole AOI. Such an image is called a Mosaic – –Mosaics will not satisfy map accuracy standards because of the displacements due to relief and camera tilt

10/7/2015 GEM Mosaic Tool can output all or part of your set of images Union Area of Interest: Defined manually or by other feature layer Map Series (USGS): User and predefined map sheets and databases used to select areas

10/7/2015 GEM – –Traditional method of mosaic creation Mosaics are made by cutting the paper prints and assembling them together, taking care that the common detail coincide as closely as possible at the match lines between adjacent photos Well constructed mosaics have the general appearance of a single photo

10/7/2015 GEM Discard Join line Cut in emulsion about 3mm outside Join Line

10/7/2015 GEM – –Types of mosaics: Uncontrolled mosaic – –Constructed from contact prints by simply matching the details of adjacent photos – –No ground control – –Individual photos are not brought to a common scale and image displacement due to relief and camera tilt remain – –Such errors are reduced by cutting out and assembling the central portions of each photo where image displacements are smaller – –Gaps or double image persists and positional errors of several millimeters are likely – –Used for natural resource surveys, reconnaissance, or for overview

10/7/2015 GEM An uncontrolled mosaic is made by simply matching like images on adjoining photographs without geographic control of the positions of the features.

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10/7/2015 GEM The scene above is a solid example of a typical uncontrolled mosaic. This is a series of high-altitude aerial photos (each about 18 km [12 mi] on a side) taken by NASA's U-2 aircraft along seven flight lines during late spring of 1972 in support of the writer's study of the geology of central Wyoming.

10/7/2015 GEM Controlled mosaic – –Prepared from photos in which the displacements errors are removed  using ortho-rectified photographs – –Control points are also used and plotted on a sheet of paper at the (approximate) scale of the mosaic – –Care is needed such that the images of the control points identified on photo paper prints are made coincident with their respective plotted positions – –Accuracy of controlled mosaics is better than that of uncontrolled mosaics but still unsatisfactory

10/7/2015 GEM – –A controlled mosaic, displays photographs that are very close to uniform scale and rectified to reduce tilt and displacement. Features on the mosaic are close to their correct positions on the map grid. – –The accuracy of a controlled mosaic approaches that of a good planimetric map.

10/7/2015 GEM Rectification – –The purpose of rectification is to produce, from nearly vertical photographs, an equivalent photograph (rectified photograph) which would have been obtained if taken from same exposure station but with truly vertical axes – –In areas of flat terrain, the main source of errors in an aerial photograph is accidental deviations of the camera axis from the vertical at the moment of exposure

10/7/2015 GEM – –Raw digital images, on the other hand, are not aligned with any conventional geographic coordinate system, and they commonly contain internal geometric distortions that result from the image acquisition process. – –These distortions can arise from tilt of the sensor plane, variations in sensor altitude, Earth curvature, lens distortion, and terrain relief, among other causes.

10/7/2015 GEM – –The Raster Resampling process (sometimes called warping or “rubber sheeting”) changes or rectifies the geometry of a raster image using the locations of ground control points that provide georeference control for the image. – –The effect of relief also affects the accuracy of images on an imagery…..The rectification concept is extended to the concept of orthophoto rectified discussed next lecture.

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