[rrd-users] Actual interpolation method

[Macckone at aol.com]

I am looking for a description of the actual interpolation method used to  
convert data points to RRAs in the specific case of 1:1 correspondence.
 
Rate generation data:
 
 
920805000:10
920805300:20
920805600:10
920805900:10
 
Generated data:


 
920805000:000 
920805300:3000 
920805600:9000 
920805900:12000

 
Example data:
920805000:000 
920805305:3100 
920805603:9030 
920805907:12070
 
RRD results:
                         <!-- 1999-03-07 05:10:00 CST / 920805000 --> 
<row><v> NaN  </v></row>
<!-- 1999-03-07 05:15:00 CST / 920805300 --> <row><v>  1.0163934426e+01  
</v></row>
<!-- 1999-03-07 05:20:00 CST / 920805600 --> <row><v>  1.9737072285e+01  
</v></row>
<!-- 1999-03-07 05:25:00 CST / 920805900 --> <row><v>  1.0098993289e+01 
</v></row>

The first data point is what would be expected.
ie. 3100/305.= 10.163934426
 
For the additional data point:
5930/298 = 19.899328859
 
Then it look like we first interpolate the data point for 
92805300 = 3049.1803279
 
That gives
5980.8197213/303=19.738678948
Which is close but still differs in the 5th decimal place.
 
If we do a full second degree interpolation:
92805300 = 3049.1803279
 
92805600 = 19.899328859*295+3100 = 8970.302013405

So 
8970.302013405-3049.1803279=5921.1216855
5921.1216855/300 = 19.737072285
 
This seems to be what is done but I have not seen a description  anywhere.
Is this indeed what is done?
First interpolating the data points and then calculating the slope?
The final data point below seems to confirm this.
Could this be added to the FAQ or the documentation some where?
 
Mack
 
Ammendum:
For the final data point.
3040/304=10
 
92805900 = 10*297+9030 =12000
12000-8970.302013405=3029.697986595
3029.697986595/300=10.098993289

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