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I am running QGIS 1.7.0 on windows, and I donwloaded a shapefile of census block group boundaries from TIGER. This file appears to be in the WGS 84 coordinate system, and when I open it in QGIS the unit of measure is degrees.

I tried re-projecting the file into the NAD 83 coordinate system, and saving it as a new shapefile, but when I open the new file it still displays a ruler in degrees. If I try to make a buffer, it interprets buffer distance as degrees.

Assuming I'm starting with a shapefile from TIGER, what's the best way to convert my unit of measure from degrees to meters?

/edit: For posterity, it seems like the following workflow make sense when dealing with US census TIGER shapefiles:
1. Download the relevant shapefile
2. Open it in QGIS
3. Click the Settings menu and select project properties
4. Select a projected coordinate system, like NAD83 / UTM zone 19N
5. Click the box Enable 'on the fly' CRS transformation
6. Click ok
7. Click the 'zoom full' button (one of the magnifying glasses)

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1 Answer 1

up vote 9 down vote accepted

You have to change the Coordinate Reference System of the project; QGIS won't do that when you load a shape file.

Go to: settings->Project Properties->Coordinate Reference System (CRS) and search for your projection.

Although WGS84 and NAD83 both refer to datums not projections. If you want to project your TIGER data, I would be inclined to use either a UTM zone, or a US-wide projection if you're dealing with large areas.

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Ok, I changed the CRS to NAD 83, and nothing changed. The unit of distance is still degrees. –  Zach Aug 3 '11 at 15:59
2  
NAD 83 is a datum, as MerseyViking said. If you change it to the relevant UTM zone (for a small area) it will work. –  Emily Aug 3 '11 at 16:03
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As I said in the reply, NAD83 isn't a projection, so it will still be in degrees: your data is mapped onto a spheroid like a globe. You need to reproject your data onto a flat plane, just like a paper map, before linear units (such as metres, feet, or miles) make sense. The Wikipedia article is a reasonable starting point for understanding the difference: en.wikipedia.org/wiki/Map_projection –  MerseyViking Aug 3 '11 at 16:07
    
Alright, thanks. I got confused between setting the CRS of the input layer and setting the CRS of the project. –  Zach Aug 3 '11 at 16:21
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Also, make sure you check the "Enable 'on the fly' CRS transformation" box if you want QGIS to dynamically reproject your layers (i.e. as you work, rather than reprojecting them and then working with the projected data). In QGIS 1.7+, this will work for both vectors and rasters; in older versions, it will only work with vectors. –  mattwigway Aug 4 '11 at 15:45
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