I have in my MySQL database both longitude and latitude coordinates (GPS data).
It's currently stored as:
column type
------------------------
geolat decimal(10,6)
geolng decimal(10,6)
Question: Do I really need a data type as large as decimal(10,6) to properly store coordinate data?
Since I have a combined index on the longitude and latitude, this index size is huge. If I can make it smaller without compromising anything, that would be great.
That depends on how precise you want your locatability to be. Obviously the larger the more precise, and the smaller the more broad your results will be. I'd suggest keeping your values larger, as it isn't really much data anyway.
I've always worked with six digits after the decimal. I used to do GIS work under a military contract and this was sufficient.
Keep in mind that it's easier to reduce data than it is to increase data. Usually, increasing data accuracy isn't even possible short of remeasuring. And remeasuring comes at a cost. Knowing nothing else of your situation or the industry, I would say capture as much data/specificity as possible.
The data that you actually use can be culled from this set. If you end up needing a higher degree of specificity, you can always recalculate without remeasuring.
Also, I'm not sure that indexing raw data is the best thing to do since it isn't a discrete set of elements. Creating a table of less accurate/smaller data points would make the indexes much smaller.
WGS84 datum are usually given as coordinates in a fully decimal notation, usually with 5 decimal places, so for latitude (-90 to +90) you could use decimal(7, 5) (-90.00000 to 90.00000), for longitude you could use decimal(8, 5) (-180.00000 to 180.00000).
.00001 gives an precision of around a meter at the equator
The DECIMAL/NUMERIC data type is a fixed precision scaled integer and both positive and negative parts of the range are always available - they do not affect the precision or scale (there is obviously storage required for it, but you don't get a choice about that for DECIMAL)
if this is for real estate do you really have so many houses that 2 bytes saved per row is going to be that noticeable? I'd keep as much precision as possible unless there was a good reason not to.
The regular GGA sentance in a NMEA Lat/lon output is only 3decimal places roughly 10m resolution at the equator. Some brands add a custom extra digit to give 1m.
4 digit deg.mm mm/1000 is also common.
If you are using high end precision RTK-GPS you might need more places to get mm precision
You can also try storing (and/or working with) your coordinates in different units. One project I worked on, all of our coordinates were in milliarcseconds stored as longs (maybe ints, it's been a couple years). This was done partially for speed and for storage space (this was an embedded system). But the same logic could apply here.