#Manual de quantum gis manual
The process of digitization is classified into manual digitizing, heads up digitizing, interactive tracing method and automatic digitizing. Digitizing is done by reading an analogue signal at regular time intervals, representing the value of that point by an integer. These integers can be converted back to analogue signal that are approximately similar to the original analogue signals. The digital signals are mainly represented in the form of sequence of integers. These bits organized in groups are known as byte. The analogue signals are variable whereas the digital format is the discrete one. This data after conversion is in the binary format, which is directly readable by computer. The process of representing an analogue signal or an image by a discrete set of its points DIGITIZATION OF TOPOSHEETS USING POINT FEATUREĪim: To digitize the given toposheet using point feature The toposheet is georeferenced as per the coordinates marked with required transformation and hence can be used for further analysis. If the coordinates are correct then the toposheet is said to be georeferenced. Compare the co-ordinates with the toposheet. To view the georeferencered toposheet, now go to add raster button and click on to zoom to layer extent. The georeferenced toposheet will be stored in TIFF format. Click on ok button and check the box load into canvas when finished.(Automatically the georeferenced toposheet is loaded into the canvas) Select the name and destination to save the file.
Now click on start georeferencing button. The points can be edited in the GCP table which can be opened through view tab. Click on add points button and add ground control points in Degrees Minutes and Seconds mode at extreme corner points in pixel zoom mode by using pan, zoom in, zoom out buttons. Add toposheet by clicking on open raster button. Select target SRS to give coordinate reference system. Select appropriate transformation type. Click on raster on menu bar and go to georeferencer and again click on georeferencer. Select programs and go to Quantum GIS Lisboa.
Using these sample coordinates or GCPs (Ground Control Points), the image is warped and made to fit within the chosen coordinate system. In case of digitize scanned maps, the coordinates can be obtained from the markings on the map image itself. These coordinates are obtained by doing field surveys - collecting coordinates with a GPS device for few easily identifiable features in the image or map. Georeferencing is the process of assigning real-world coordinates to each pixel of the raster.
Aim: To georeference the given toposheet by using georeferencer.
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