# Interpolating between lines

1. ## Interpolating between lines

Hi all,

I am trying to digitize a performance graph for a single engine aircraft.

I have used webplotdigitzer and ploty to extract the x,y values and find the equation for the lines.

I am having trouble finding an accurate interpolation between the pressure altitudes. I've tried FORECAST.LINEAR and TREND but they are not accurate enough.

What would you recommend on doing?

Screenshot (308).png

Thanks.

2. ## Re: Interpolating between lines

1. We can do nothing with a picture. We need an Excel file.

2. Whch lines? There are too many of them and no clear explanation as to where we should be looking and what you actually expect to see.

3. ## Re: Interpolating between lines

Sorry Glenn,

Brain was a little tired last night and I rushed this post before I nodded off.

Basically I am focusing on the far left side of the graph. The 7000ft, 6000ft, 4000ft, 2000ft and Sea Level lines.

I have two inputs:
1. Temperature
2. Pressure altitude

Temperature determines the x-axis.

Pressure altitude determines the y-axis.

What I am having trouble with is interpolating between the Pressure altitude lines.

For example:
A temperature of 0°C that gives an x value of 20.
A pressure altitude of 3650 gives a y value of ????.

I have attached a workbook which hopefully shows what I am trying to do.

Thank you.

4. ## Re: Interpolating between lines

Not sure exactly what you looking for.
Does this interpolation helps?

= 8.61662698516537*EXP(1.8225405785263*N48/10000)

5. ## Re: Interpolating between lines

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6. ## Re: Interpolating between lines

The thread title talks about interpolation, but most of the algorithm appears to be using a regression type of algorithm. After regressing parameters in J48:L52, you then calculate Y at each PA, then a linear regression algorithm (FORECAST.LINEAR() and TREND() both use the same basic linear regression algorithms) to find the final value.

If you want to do linear interpolation on the values in N48:O52 to get the final value and are not limited to Excel as a spreadsheet, Gnumeric is a spreadsheet (native only to Linux OS) that has a built in INTERPOLATION() function that makes short work of linear (an other) interpolation problems.

If you must use Excel and you really want more of an interpolation algorithm, my preferred approach to setting up an interpolation problem in Excel: https://www.excelforum.com/excel-for...ml#post5262186

If I'm understanding what you are trying to do, see if any of that helps. If I'm not understanding, help us understand what you are trying to do and the algorithm you want to use to do it, and we should be able to help you program that algorithm into the spreadsheet.

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