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Lab 6 Georectifying

Lab 6 Georectifying - 44:005 Foundations of GIS Lab 6...

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44:005 Foundations of GIS Lab 6: Georectification The objective of this lab is to introduce you to the concept of georectification . In Lab 3, we used projection tools in ArcMap to change the projection of one layer so it would line up with another layer in our project. In this lab, one of the images we want to use doesn’t have spatial reference information, so it can’t be projected. Instead, we have to use the process of georectification to align the two images so they can be used in our project. Overview of the exercise… Background on georectification Placing control points RMS (Root Mean Square) Error Permanently transforming the image I. Background Raster data for GIS projects often comes in the form of scanned maps, aerial photos and satellite images that are initially lacking spatial reference information. If you load a dataset lacking spatial reference information into ArcMap, you will get a warning message that the layer is lacking spatial reference information and can be drawn on the screen, but not projected to a coordinate system. To use this dataset in a GIS, you must first align the map or image with a coordinate system. You can do this using another image of the same area that is already aligned with a coordinate system. In the exaggerated illustration below, a georectifyd image (A) can be used to georectify another image (B). Georectification image B will produce an image very close to image A. After georectification, image B will be stored in the same coordinate system as image A. The process of georectification relies on the coordination of a small set of points on the image to be georectifyd with corresponding points on another image that is already geographically referenced. By “linking” a set of points on the image with those same locations in the geographically referenced data, you create a mathematical transformation that converts the location of all points in the unregistered image to their correct geographic location. Now that the two images are aligned, they can be used together in a GIS for mapping and spatial analysis. The corresponding points used to link the two data layers are called control points (shown as little white crosses in the image below). 1
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A. Georectifyd image B. Image to be georectifyd Image from B. Zitova´, J. Flusser, Image registration methods: a survey, Image and Vision Computing 21 (2003) 977–1000 II. Placing control points in ArcMap In this exercise we will be georectifying an aerial photo of Iowa City (ungeorectified.tif). This image has no spatial reference information stored with it (confirm this by reading about its coordinate system information in data frame properties), so it cannot be projected as we did in Lab 3. Instead, we will use a previously georectified topographic map of Iowa City (georectified.tif) to georectify the aerial image. 1.
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  • Fall '11
  • DavidBennett
  • Geographic coordinate system, Coordinate system, control points, ArcMap, Georectification, spatial reference information

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Lab 6 Georectifying - 44:005 Foundations of GIS Lab 6...

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