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I've created a point data layer in QGIS using a .png image as a template. Of course the .png had no projection, and when creating the new shapefile I chose the WGS_84 coordinate system. Now, I need these data points to 'match up' with a (correctly projected) shapefile of Ireland in ArcMap 10.2. When adding the newly created data, I got a message stating something along the lines of the shapefile not being the same extent as the other data and that projecting the data may result in unexpected behavior. So the data points have been projected in some ocean judging by the basemap, and using the Data and Projections > Batch Project tool to change the PCS to TM65 (the Irish National Grid) has not rectified this.

Is it possible to change the coordinates of each point (there are over 100 at this stage...) to align with a shapefile of Ireland, or do I just have to start from scratch and create the points again, this time over the shapefile? Also, would I be able to align .png files with the Ireland shapefile to prevent this predicament for the next batch of data points I am going to create?

  • How did you create the point layer? Did you digitize the points on top of your .png? – a1234 Mar 19 '15 at 15:52
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    I would have an accurate projection for your raster image, prior to digitizing the points. – a1234 Mar 19 '15 at 16:08
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    Yes, you can bring in a .png, georeference the image in your coordinate system, giving it accurate spatial reference. Then create your point layer. I haven't used QGIS much, I primarily use ArcMap, but I'm sure there is a QGIS equivalent to this process. – a1234 Mar 19 '15 at 16:14
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    Correct. If the points were digitized on top of something with no spatial reference, in a way you are carrying that over to your point layer. – a1234 Mar 19 '15 at 16:25
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    To further arzola's last comment, not only was it carried over, it was actually compounded by the fact you assigned it a spatial reference when it didn't have one. Project converts between known coordinate systems. If you assign the wrong CRS to data, then Project is going to give an incorrect result because it doesn't know any better. As mkennedy mentions, you might be able to warp your points into place, but the best thing to do is georeference the png to Ireland shapefile before digitizing the points (which also lets you stay in one CRS from the start and not worry about converting). – Chris W Mar 19 '15 at 23:23
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Maybe you don't have to redigitize your points. Once you are able to reference your png you can also transform the points you digitized so far.

First you have to check, which projection your png file resembles. Depending on the projection that had been used to produce your png ireland could look quite differently. Next thing you need are at least two pairs of control points telling you the source (digitized) and target (correctly georeferenced) coordinates. Now, if you're good at maths it won't be hard to give your png file a world file that tells it where it belongs to on the map and to get the parameters of the transformation to beam your points onto the image again.

There are many ways to georeference an image. I'd suggest to construct a world file, you can do that independently of a GIS program. To construct the world file you have to find out the resolution of your pixels (width and height; height preceded by a minus sign ) and the real coordinates of the center of the topleft pixel of your png. You can find out by calculating the differences in the control point coordinates (just make sure that you really used pixel coordinates, otherwise you have to divide appropriately).

To transform your points you can recalculate the shapes directly (I don't know if and how this can be done in QGIS without writing a script), write them into two separate fields and load as an event theme or use the SAGA transform shapes routine. Transformation again needs a shift parameter of the origin of your coordinates (where goes the 0,0?), and scale factors dx and dy (could be the same as in your world file, but both positive). Probably it would be easiest to do it together once you know the values of your control points.

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    The tools in ArcGIS for these steps are to use the georeferencing toolbar for the png file (you'll need to have some control points whose CRS and coordinates are known) and the spatial adjustment toolbar for the points. You may find that the latter isn't working that well and then decide to redigitize the points. – mkennedy Mar 19 '15 at 22:50
  • Thanks for the advice. In the end, long. and lat. values were added with my supervisor, though i'll look into how to create world files for future reference. – Milo Mar 20 '15 at 18:22

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