I'm trying to merge a census shapefile of New Orleans with a health outcomes dataset by address with QGIS. I have geocoded the addresses and added them as a layer in QGIS but don't know on which variable within the street level city shapefile to merge them. The dataset has long/lat but the shapefile does not.
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Or maybe I don't need to merge them but their coordinates and scales are very different and i don' tknow how to rectify that– user27464Commented Feb 27, 2014 at 18:02
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1Do you want to add the census shapefile fields to the address points? To do that, use Vector->Data Management->Join attributes by location– evv_gisCommented Feb 27, 2014 at 21:56
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It sounds like they may be using different coordinate systems. One may be lat/lon but the other is in state plane or UTM.– mkennedyCommented Feb 27, 2014 at 22:25
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1 Answer
What you are trying to achieve is called a spatial join. You want to join the attributes of the database to the census shapefile, which holds the geographic/spatial data.
Since you are working with a Census Data Shapefile, I am assuming that it is either a County or Tract Polygon layer. And since you talked about geocoding your health outcomes database, I am assuming you are working with point data for that layer. If you can indeed see the points of your geocoded database points in QGIS (aka they have sufficient spatial information to be displayed) and you want to join them based upon location in relation to polygon layer (census) then you can do as evv_gis suggested above and do Join attributes by location. A regular Spatial Join by Attribute is usually implemented when you have a linking key or column that is shared between the two separate tables that the join is based upon. The Spatial Join by Location is used when no linking key exists and only the geographic location of one in relation to the other can be used to join data to the table.
It isn't clear from your question which way you want the join to go, either the points onto the polygons or vice versa. So based upon your intent, you would decide which way to go.
The scale and lat long coordinates may be slightly different because they are different vector formats, one point and the other polygon. Check out an example of this polygon dataset that is different than just simple latitude, longitude.
