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I am a very definite QGIS newbie. I have been creating a map using files that had been used in ArcGis. Each time I wanted to add a new vector layer I opened an .shp file and specified CRS as WGS 84. The resulting layers look fine but when I try to use the "measuring tool" to measure line length I get an error message as follows:

This map is defined with a geographic coordinate system (latitude/longitude) but the map extents suggests that it is actually a projected coordinate system (e.g., Mercator). If so, the results from line or area measurements will be incorrect. To fix this, explicitly set an appropriate map coordinate system using the Settings:Project Properties menu.

  • any advice on how I might determine what might be an "appropriate map coordinate system" ?

  • I am also wondering if I should have also opened the corresponding .shx, .sbx, .dbf, .sbn and .qix files also ?

Thanks

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Edward, I'm going to answer your 2nd question 1st. "Do I need to open all the other files?" The .shx, .sbx, .dbf etc.... all hold information that compliments the .shp file you have. When you open the .shp in QGIS, it will automatically read the relevant information form each of those files. It is very important that if you move the .shp to another folder, you move all those files with it, if you re-name the .shp also make sure you rename all these other corresponding files to the same name. They all make up the .shp file. If you only had the .shp file and nothing else, you wouldn't be able to open it. So no, you do not need to also add all these files into QGIS as it is already reading data from them.

The error you're getting, as stated by your post above, is because you're using a geographic coordinate system (wgs84) rather than a projected system. In very simple terms, a projected system is where you project an image of the earth onto a flat 2D piece of paper. Imagine you have a hollow model of the earth with a light bulb in the middle. The oceans are see-through and the land won't let any light through. If you hold a sheet of paper flat above one of the sides of this model, and turn on the light, the land will cast shadows on the paper, essentially "projecting" an image on to the paper. In other words, projecting a 3D earth onto a 2D sheet of paper. Taking a measurement from the sheet of paper is easy because it's flat. This is a very simplistic explanation, but I hope it helps you understand what a projected system is.

Before we can answer your 1st question about determining the best projection for your data, which part of the world is your data for?

  • Thanks Dan your answer was most helpful. The data is from Ireland. I am now using TM75 (ESPG 29903) and the measuring tool is working fine. However, I have encountered another measuring issue about which I will post a question shortly. Your help would be most welcome again. – Edward Nov 7 '13 at 23:04

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