RE: [OpenMap Users] Floats Vs. Doubles

I may have been a bit hasty. I'll try to do something more accurate tomorrow.
If you only have 6 digit accuracy that should be good to about 5 feet. Do you have data that's more accurate than that?

k
At 03:45 PM 12/4/2003 -0700, Martin Chapman wrote:
>Maybe my problem is casting the float then. When ever I cast a double
>that has more than 6 decimal places to a float the number gets truncated
>to 6 places. I've stepped through my code and seen this happen. Even
>trying to create a float from a hard coded number that has more than 6
>decimals it gets truncated to six. The rounding error causes my poly
>geometry not to but up against each other therefore resulting in gaps in
>my imagery cuts. So I don't think the float is keeping the accuracy you
>say it has. At least not for me. The results are visible when you zoom
>way in on the map. Any ideas?
>
>Martin
>
>-----Original Message-----
>From: Ken Anderson [mailto:kanderson@bbn.com]
>Sent: Thursday, December 04, 2003 3:40 PM
>To: Don Dietrick
>Cc: Martin Chapman; openmap-users@bbn.com
>Subject: Re: [OpenMap Users] Floats Vs. Doubles
>
>
>In JScheme, you can do
>(define (epsilon x) (if (not (= (+ 1.0 x) 1.0)) (epsilon (/ x 2.0)) to
>get a quick estimate of floating point epsilon:
>> (epsilon 1.0F)
>5.9604645E-8F
>So, say we have 7 places, and at the equator, one degree is 60 nautical
>miles, and one nautical mile is 6076 feed. This gives us a float
>accuracy of
>> (* (* 60 6076.0) 1.0e-7)
>0.036455999999999995
>or 0.44 inches.
>
>Do you really have data that is more accurate than that?
>
>k
>At 05:03 PM 12/4/2003 -0500, Don Dietrick wrote:
>>Hi Martin,
>>
>>On Dec 3, 2003, at 11:55 AM, Martin Chapman wrote:
>>
>>>Don,
>>>
>>>Why did the architects of OpenMap choose to use floats versus doubles
>>>to store point data?
>>
>>It was a design decision made in 1996, and we chose to save memory over
>
>>the precision of 7-10 feet, or whatever precision the float gives. It
>>depends on the latitude, but I can't remember exactly what the number
>>is at the equator.
>>
>>Most of the data sources we dealt with at that time were world-wide
>>databases, and the float accuracy was good enough.
>>
>>>Sometimes I have a precision problem when plotting geometry on the map
>
>>>because my source data is double value and the coordinates get
>>>truncated. This is a problem when dealing with high resolution
>>>satellite imagery that requires sub-meter accuracy, which is nine
>>>decimal places I think. Do you know how many decimals a float can go
>>>to? Is there a way to get around this problem?
>>
>>Doesn't the number of decimal places depend on the number you are
>>trying to represent?
>>
>>The only way I know to get more precision is to modify the Projection
>>libraries and OMGraphics to use doubles in addition to floats. Big
>>job.
>>
>>- Don
>>
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