I am new to GIS use and I have a plan (scanned from paper, old fashion) with a grid of 5x5 meters cells and a GPS point in the middle. I wish to georeference my plan, but I only have one known point. I was wondering if there was a calculation to find out what coordinates are associated to the points on the grid?
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What kind of plan? Can you post a picture of it? Otherwise one point is not enough as you said. But with this one point you can move the picture to the right area and georeference on a aerial picture(OpenLayers extension for example) and try to find other point you can identify.– MatteCommented Apr 5, 2016 at 18:44
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I am not sure if I can post a picture as it is not my own data, but it is the plan of an archaeological site. The process to draw such plan is to define a grid on the ground. Here square mesure 5m x 5m, and everything found within one square is drawn using mesures from the square border. In the end, all squares are brought together in the final plan. Therefore, from the GPS point, I know to the cm where things are...but not in coordinate.– Laurence FerlandCommented Apr 5, 2016 at 23:18
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2 Answers
You can set up a local coordinate system, based on the origin of your local grid, ideally your GPS reference point.
Follow my advice here:
The first thing you should do is to check what acual scale your map has after you scanned it. When you know that your grid has 5x5m you can count the pixels that are within in the 5x5 grid to get your scale. (consider to count in x and y direcetion as your scan might be a bit distorted) You can count the pixels "by hand" or for example within GIMP you got a measure tool that tells you the amount of pixel for a given line(looks like a pair of compasses). With this information you can get new points by measuring distances from you GPS-Point and can translate them into coordinates by your on paper-scale.
