[postgis-users] Float 8

[Chris Hermansen]

Here's an example that I can think of.

Suppose I digitize a bunch of linework from some old manuscripts that
were hanging in a file cabinet, and they stretched.  My control points
were to the north of the data I digitized.

When I view the data that I digitized in conjunction with other existing
data, I can see that it's about 100 metres south of where it should be.

For this I would consider using ST_Transform() to shift my digitized
data north by 100 metres.

Something like ST_Transform(the_geom,0.0,100.0,0.0)

Does that help?

Bob Pawley wrote:
> Hi Chris
>
> How is the float 8 number generated - manually??
>
> Seems to me one should be able to generate it relationally. Perhaps
> that's the point I'm missing?
>
> Bob
>
>
>
>
>
> ----- Original Message ----- From: "Chris Hermansen"
> <chris.hermansen at timberline.ca>
> To: "PostGIS Users Discussion" <postgis-users at postgis.refractions.net>
> Sent: Thursday, May 22, 2008 2:45 PM
> Subject: Re: [postgis-users] Float 8
>
>
>> Hmm, the form it takes?  Not sure I understand you.  Let me try
>> nevertheless.
>>
>> A float8 is a single number.  It's 64 bits of floating point, part of
>> which is exponent and part of which is mantissa.  The exponent is
>> understood as a power of two.  The most common double precision floating
>> point representation in use today represents numbers to about 14 digits
>> of accuracy in the range of 10^-308 to 10^308, negative and positive.
>> There are other special numbers like ± infinity and NaN (not a number).
>>
>> Most computers use the IEEE 754-1985 standard for floating point.  You
>> can read all about this at
>> http://en.wikipedia.org/wiki/IEEE_floating_point
>>
>> A concrete example of a float8 number is 1.4976931341623151×10^3
>>
>> What you are looking at specifically -
>> ST_Translate(geometry,float8,float8,float8) - takes a given geometry and
>> "translates" (ie moves relative to the origin) by the three float8
>> offset values you provide.  You might think of this as a shift.
>>
>> Let's say you called ST_Translate() on a point (x,y,z) with offset
>> values (a,b,c).  Then your translated coordinates would be:
>>
>> x' = x + a
>> y' = y + b
>> z' = z + c
>>
>> ST_Translate() is one of the three components of an affine
>> transformation, the other two being ST_Scale() and ST_RotateZ().
>>
>> Does this help?
>>
>> Bob Pawley wrote:
>>> Any suggestions on the form it takes (integer, cooridinate, binary)
>>> and how it is used with ST_Translate???
>>>
>>> Bob
>>>
>>> ----- Original Message ----- From: "Chris Hermansen"
>>> <chris.hermansen at timberline.ca>
>>> To: "PostGIS Users Discussion" <postgis-users at postgis.refractions.net>
>>> Sent: Thursday, May 22, 2008 1:48 PM
>>> Subject: Re: [postgis-users] Float 8
>>>
>>>
>>>> float8 is double precision, about 14 digits of precision to be precise
>>>> :-) plus a bigger allowable exponent than float4 or single precision.
>>>>
>>>> Bob Pawley wrote:
>>>>> Hi
>>>>>
>>>>> Would someone please tell me what is meant by float 8 in this
>>>>> function? I know they refere to x, y and z coordinates. (An example
>>>>> would help)
>>>>>
>>>>> ST_Translate(geometry, float8, float8, float8)
>>>>>
>>>>> Bob
>>>>> _______________________________________________
>>>>> postgis-users mailing list
>>>>> postgis-users at postgis.refractions.net
>>>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>>
>>>>
>>>> -- 
>>>> Regards,
>>>>
>>>> Chris Hermansen         mailto:chris.hermansen at timberline.ca
>>>> tel+1.604.714.2878 · fax+1.604.733.0631 · mob+1.778.232.0644
>>>> Timberline Natural Resource Group · http://www.timberline.ca
>>>> 401 · 958 West 8th Avenue  · Vancouver BC · Canada · V5Z 1E5
>>>>
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>>
>>
>> -- 
>> Regards,
>>
>> Chris Hermansen         mailto:chris.hermansen at timberline.ca
>> tel+1.604.714.2878 · fax+1.604.733.0631 · mob+1.778.232.0644
>> Timberline Natural Resource Group · http://www.timberline.ca
>> 401 · 958 West 8th Avenue  · Vancouver BC · Canada · V5Z 1E5
>>
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-- 
Regards,

Chris Hermansen         mailto:chris.hermansen at timberline.ca
tel+1.604.714.2878 · fax+1.604.733.0631 · mob+1.778.232.0644
Timberline Natural Resource Group · http://www.timberline.ca
401 · 958 West 8th Avenue  · Vancouver BC · Canada · V5Z 1E5