[postgis-devel] about interger storage

[Paul Ramsey]

Just to be the contrarian, can't I use a system of start point +
offsets to obtain both high relative accuracy and positioning to
within a mm?

Of course, the same trick can be used in db, to even greater effect, so...

P.

2008/5/9 Martin Davis <mbdavis at refractions.net>:
> 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
>
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