Question

I am to compress location data (latitude,longitude, date,time). All the numbers are in fixed format. 2 of them (latitude,longitude) are with decimal format. Other 2 are integers.

Now these numbers are in fixed format string.

What are the algorithms for compressing numbers in fixed format?
Is number only compressions (if there any) better than string compression?
Should I directly compress string without converting it to numbers and then compress?

Thanks in advance.

Answer 1

This is one of these places where a little theory is helpful. You need to think about several things:

• what is the resolution of your measurements: 0.1° or 0.001°? 1 second or one microsecond?
• are the measurements associated and in some order, or tossed together randomly?

Let’s say, just for example, that the resolution is 0.01°. Them you know that your values range from -180° to +180°, or 35900 different values. Lg(35900) ≈ 16 so you need 16 bits; 14 bits for -90°–+90°. Clearly, if you’re storing this kind of value as floating-point, you can compress the data by half immediately.

Similarly with date time, what’s the range; how many bits must you have?

Now, if the data is in some order (like, samples taken sequentially aboard a single ship) then all you need is a start value and a delta; that can make a big difference. With a ship traveling at 30 knots, the position can’t change any more that about 0.03 degrees an hour or about 0.0000083 degrees a second. Those deltas are going to be very small values, so you can store them in a very few bits.

The point is that there are a number of things you can do, but you have to know more about the data than we do to make a recommendation.

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Update: Oh, wait, fixed point strings?!

Okay, this is (relatively) easy. Just to start with, yes, you want to convert your strings into some binary representation. Just making up a data item, you might have

040.00105.0020090518212100Z

which you could convert to

|  4000            | short int, 16 bits |  
| 10500            | short int, 16 bits |  
| 20090518212100Z  | 64 bits            |

So that’s 96 bits, 12 bytes versus 26 bytes.