Yes, I do think it helps.
Thanks for your time.
Bob
----- Original Message -----
From: "Chris Hermansen" <[EMAIL PROTECTED]>
To: "PostGIS Users Discussion" <[email protected]>
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"
<[EMAIL PROTECTED]>
To: "PostGIS Users Discussion" <[email protected]>
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"
<[EMAIL PROTECTED]>
To: "PostGIS Users Discussion" <[email protected]>
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
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Regards,
Chris Hermansen mailto:[EMAIL PROTECTED]
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:[EMAIL PROTECTED]
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:[EMAIL PROTECTED]
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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