[postgis-users] Float 8

[Bob Pawley]

Yes, I do think it helps.

Thanks for your time.

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 4:13 PM
Subject: Re: [postgis-users] Float 8


> 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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>>>>
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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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>>
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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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