[postgis-devel] about interger storage

[Martin Davis]

Further to this issue:

Perhaps the most important reason to choose double-precision storage
over integer storage is this:

DP provides substantially more bits of precision for representing
values. Integer (32-bit) gives about 8 decimal digits of precision. This
is roughly enough to store values over a 1000 km range to mm precision.
DP on the other hand provides 16 decimal digits of precision - more than
enough to map the globe to mm precision.

1000 km to 1 mm may sound like a lot, but it isn't really. I work in a
jurisdiction which barely fits in this range. In many of our databases
we like to keep an extra digit or two of precision to give some room for
mathematical manipulation - so we need more than 9 digits.

You may also say that 1 mm in 1000 km is much more accuracy than any
real data provides. That is probably true, but the key issue is the
relative accuracy requirements. The datastore has to provide a
coordinate space which can support the maximum required relative
accuracy. And it's quite common to have data which has fairly high
accuracy relative to nearby features. (Or at least, a high relative
precision is required in order to distinguish feature coordinates)

Martin Davis wrote:
> Why do you think integer storage will provide better accuracy,
> integrity, and speed?
>
> - accuracy is a function of your data
> - if you are concerned about *precision*, then just having the database
> automatically force everything into a fixed precision model doesn't
> really address all concerns. You really need to round your data
> yourself, and deal explicitly with any data collapse issues
> - speed - well, in theory yes, since you are shipping half the data over
> the wire. But have you measured this? There's a lot more involved than
> raw wire speed.
>
> A major downside of integer storage IMO is the need to specify your data
> extent explicity up front. And the need to constantly transform between
> integer and the external coordinate system (in floating point) is a
> major pain.
>
> xie jiong wrote:
>   
>> Hi everyone:
>> Does postgis support interger storage? We are thinking about
>> conversion from floating point real¨Cworld units to positive integer
>> units for storage in the database to provide better data accuracy,data
>> integrity,and processing speed as ArcSDE do. Are there any possible
>> and difficulties of doing so? Dose Geos support it? As far as I know,
>> spatial databases such as Oralce spatial don't use it but I think it
>> will really improve performance and low down space. Any idea?
>> Jiong
>> ------------------------------------------------------------------------
>>
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>
>   

-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022