The image below shows an aerial photograph and digital deposit map data of a local school campus. It is a scanned image taken from a GIS system. The original cadastral (lot boundaries) were vector (CAD), but because this is a raster (pixel) based image, there is no vector information in it, nor is there any sense of size. The task is to produce an accurate full size CAD drawing showing the location of important buildings in the campus.

School

Start gCADPlus, beginning with your template drawing that contains at least one layout view with your own logo.

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We suggest that you use the Draw>Insert Raster tool and insert a copy of this scanned image into the gCADPlus environment. It is useful to insert the image at a scale of 1.0 (taking the scale image at insertion option off) because if the original scanning has been done carefully, there may not be much need to re-scale the image to match the dimensions of the 'real world'.

Note that not only can the option to scale the image on entry be turned off, it is also possible to change the resolution of the scanned image at insertion. A resolution of 150 dpi rather than the 300 dpi shown below should be ample to allow accurate copying, but we used 300 in the example below.

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The next step is to scale (calibrate) the image until and enquiry of the drawing using the distance command reveals a true length as measured on the ground. To do this find an 'object' of known length. As an example, the typical length of a parking bay is 5500 units and that could be used to scale the drawing. It is best to take the longest 'object' on the image. in this case, the basketball court which should measure 28650 units. The DIST command shows it measures 2789. so we need to scale by 28650/2789.

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Use gCADPlus calculator on the Tools drop down menu to determine the required scale factor - 10.28

Use SCALE > Select all > Base point 0,0 >10.284

Now a DIST command shows true values.

Make a new layer

Create a new layer for the vector information you will lay on the top of the drawing. make that layer current.

Now trace the outline of buildings. In this case, starting with the lot boundaries (in cyan) is likely to help orientation as you are tracing.

It is useful to rotate the image and use the ORTHO switch on to make tracing much easier.

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It might be useful to use the Google Earth tool and include a site location with road names into the drawing.

Trace aerial photograph