Validate three digits before decimal and after decimal point six digits
You're better off using a DECIMAL(9,6) data type in the database.


And then you've validate with a simple numeric BETWEEN

However, if you insist on a character type, use CHAR instead of NCHAR - you're only accepting periods and numbers, so there's never any "Unicode"; cut your space use in half.


And then you could validate with something like

a.latitude LIKE '[0-3][0-9][0-9].[0-9][0-9][0-9][0-9][0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-3][0-9][0-9].[0-9][0-9][0-9][0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-3][0-9][0-9].[0-9][0-9][0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-3][0-9][0-9].[0-9][0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-3][0-9][0-9].[0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-3][0-9][0-9].[0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-3][0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-9][0-9].[0-9][0-9][0-9][0-9][0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-9][0-9].[0-9][0-9][0-9][0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-9][0-9].[0-9][0-9][0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-9][0-9].[0-9][0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-9][0-9].[0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-9][0-9].[0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-9].[0-9][0-9][0-9][0-9][0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-9].[0-9][0-9][0-9][0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-9].[0-9][0-9][0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-9].[0-9][0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-9].[0-9][0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-9].[0-9]' ESCAPE '~'
OR a.latitude LIKE '[0-9]' ESCAPE '~'

Note that doing this with character types is both annoying and not nearly as performant.



Note that the leftmost digit is limited to 0-3, since you'd never have a latitude or longitude starting with a 4 or higher (I'm assuming degrees).

Since you're on SQL Server 2008 R2, you may also want to read the Stackoverflow question Why use the SQL Server 2008 geography data type?.  A very brief summary is: DECIMAL(9,6) is significantly smaller, and for a simple SELECT Lat, Long is significantly faster.  However, to do accurate proximity searches (which is often why coordinates are stored), distance calculations (also often why you need them), or areas and the like, the geography type is much simpler and almost certainly more accurate.