Imagine that you are writing a program where you need to display the location of each city and town on top of a raster basemap. You dutifully obtain a nice raster datasource to use for the basemap, and then search the Internet for a source of city and town data. You choose the National Geospatial Intelligence Service (NGIS) website to download a database of place names, which you then draw onto your map. This database includes, among other things, the latitude and longitude of each place name:
|
Location |
Latitude |
Longitude |
|---|---|---|
|
Abache |
7.3551 |
7.6407 |
|
Abacheke |
5.50372 |
6.729519 |
|
Abacher |
13.816667 |
20.816667 |
|
Abacheri |
14.183333 |
41.5 |
|
Abachi |
7.3551 |
7.6407 |
|
...and so on |
So far so good, but when your program is complete, the locations look suspiciously regular when the user zooms in on your map:
If you were to draw a grid on top of this map, you can see exactly what the problem is:
As you can see, the locations are regularly spaced—despite there being lots of precision...