# [rrd-users] Actual interpolation method

Macckone at aol.com Macckone at aol.com
Wed Apr 8 02:22:16 CEST 2009

```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

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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