Geo Location

How accurately should I store latitude and longitude?

decimal  degrees    distance
places
-------------------------------  
0        1.0        111 km
1        0.1        11.1 km
2        0.01       1.11 km
3        0.001      111 m
4        0.0001     11.1 m
5        0.00001    1.11 m
6        0.000001   0.111 m
7        0.0000001  1.11 cm
8        0.00000001 1.11 mm

ref : https://en.wikipedia.org/wiki/Decimal_degrees#Precision

What is the ideal data type to use when storing latitude / longitude in a MySQL database?

In SQL database, Latitude and Longitude data types should use DECIMAL instead of FLOAT.

E.g. DECIMAL(9,6) depending on the precision required

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OR

Use MySQL's spatial extensions with GIS.

Another practical way is to use integer or number.

We store latitude/longitude X 1,000,000 in our oracle database as NUMBERS to avoid round off errors with doubles.

Given that latitude/longitude to the 6th decimal place was 10 cm accuracy that was all we needed. Many other databases also store lat/long to the 6th decimal place.

Thus an Int data type could also serve the purpose. Say 168.123456 * 1 000 000 = 168 123 456 is still much less than the signed integer limit which is around 2 billion (2 * 10^9).