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I start a project in QGIS. I set the CRS to anything (in this case I use EPSG=26918 a NAD83 datum). I also enable on the fly projection. Then I import a vector. In this case a kml, which I believe is a CRS of WGS84. Then I use openlayers and add a google satellite image. Everything looks good and lines up well. I then open a composer window and add a map and a scalebar. When I check to see if the scalebar is accurate I find that it is not.

If I go back to the project and check the measurements with the measurement toolbar I find that the measurement is accurate when the ellipsoid button is checked and inaccurate when the ellipsoid button is not checked. The measurement that I get when the ellipoid button is not checked is the same as I get in the composer window when I use the scalebar to figure out a length.

I guess I have a sense of why this happens. But I want to know how I can get an accurate scalebar in my composer window when I am using a google satellite image?

Thanks, David

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1 Answer 1

Openlayers plugin only works correct with EPSG:3857. But this is not a projected CRS with metres as units. Instead it uses sphere calculations, with units differing from real metres the more to the poles you come. EPSG 26918 will give you correct measurement, but no Google picture.

So you have to decide if you want googles background or correct measurement.

What you can do is make a screen picture of the google layer. This will automatically be georeferenced. You can then switch the CRS to UTM, and load that picture instead of Google Openlayer plugin.

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Hi Andre. Thanks! Recall that it isn't exactly a correct measurement I am looking for but a correct scalebar in my printer composer window, which then I will place in a document. It is critical that the scalebar is accurate. My guess is many people are simply making maps with google satellites but unknowingly have inaccurate scalebars. Your work around sounds good but I hear the quality of the final map suffers from low resolution. Can someone tell me a good way of doing this (i.e. make a screen picture that is automatically georeferenced and has has a high resolution.) –  David Jan 7 '13 at 15:07
    
You can zoom in and take several screen copies instead of just one. If the reprojection leads to ugly black borders, you can stitch the jpgs together with GDAL buildvrt before reprojecting. –  Andre Joost Jan 7 '13 at 17:32

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