[OSM-dev] Binary OSM; the first pass encoder

[Tomas Kolda]

Integer has better precision if you are far from zero, because floats 
use only 24 bits for significant, other bits is for sign and exponent. 
So 32 bit integer is better if you know extends (-180, .... 180). Min 
and max of int is -2.1e9 to 2.1e9. So if you multiply longitude and 
latitude by 1e7 you get almost maximum from 32bit int in all range (from 
-1.8e9 to 1.8e9).

It is also faster to process. And if you are doing some difference 
compression, it is easier to do with ints.

Tomas

Marcus Wolschon napsal(a):
> Hello Robert,
>
> as far as I understand it the OSM-databases store the location
> in long integers anyway. Thus there is no precision to be gained
> from using  floats. You only loose storage-space, computation-time
> and native functionality on platforms that do not have native floating-
> points (like many cellphones, reflashed navigation-devices or
> all mobile J2ME java-platforms).
>
> Marcus
>
> 2008/11/12, Robert (Jamie) Munro <rjmunro at arjam.net>:
>   
>> Floats only allow more precision as the location being considered tends
>> towards 0,0 latitude and longitude. For the general case, using an
>> integer with the same number of bits of storage will always be more
>> precise, particularly at the extremes.
>>
>> As others have said, 32 bits gives errors of less than 10mm on the
>> equator, with even greater accuracy as you move away from it. This is
>> around 1000 times more accurate than the data in OSM can be reasonably
>> recorded at with standard GPS equipment.
>>
>> Robert (Jamie) Munro
>>     
>
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>   

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