I want to use GeoDjango to do basic location searches. Specifically I want to give the search function a ZIP code/city/county and find all the ZIP codes/cities/counties within 5mi, 10mi, 20mi, etc. I found the following paragraph in the documentation:
Using a geographic coordinate system may introduce complications for the developer later on. For example, PostGIS does not have the capability to perform distance calculations between non-point geometries using geographic coordinate systems, e.g., constructing a query to find all points within 5 miles of a county boundary stored as WGS84. [6]
What does this exactly mean if I want to use PostGIS and to be able to do the searches described above across the USA? The docs suggest using a projected coordinate system to cover only a specific region. I need to cover the whole country so this I suppose is not an option.
Basically in the end I want to be able to find neighbouring ZIP codes/cities/counties given a starting location and distance. I don’t really care how this is done on a technical level.
Also where would I find a database that contains the geographic boundaries of ZIP codes/cities/counties in the USA that I can import into a GeoDjango model?
UPDATE
I found a database of that contains the latitude and longitude coordinates of all ZIP codes in the USA here. My plan is to import these points into a GeoDjango model and use PostGis to construct queries that can find other points within x miles from a given point. This gets around the issue raised in the documentation because all the ZIP codes are treated as points instead of as polygons. This is fine for my use case because perfect accuracy is not something I care about.
The good: the data file is free
The bad: this data is from the 2000 census so it is quite dated
The somewhat hopeful: the United States Census Bureau conducts a census every 10 years and it is almost 2010
The conclusion: it’s good enough for me
To get around the limitation in the quote, you can just take the centroid of the zipcode region provided by the user, and then from that point find all zipcode regions that intersect a 5, 10 or whatever mile circle emanating from that point. I’m not sure how that would be achieved in geodjango, but with postgis it’s definitely possible.
The limitation you quoted basically says you can’t write a query that says “give me all points that are within 5 miles on the inside of the border of Ohio.”