How to find the intersection point of two lines exactly, the lines are generated using some data. The x-axis is on the logarithmic scale and the y-axis is on a linear scale.

    sd1 = Subdivide[0.001, 0.5, 300];
    sd2 = Subdivide[0.55, 2, 300];
    sd3 = Subdivide[2.2, 100, 300];
    sd = Flatten[{sd1, sd2, sd3}];
    sd = sd[[1 ;; All ;; 8]];
    data1 = Transpose[{sd, a}];
    data2 = Transpose[{sd, b}];
    ListLogLinearPlot[{data1, data2}, Joined -> True, PlotRange -> All]

Here I show a tricky direct solution using Graphics`Mesh`FindIntersections which doesn't need interpolation:

pic=ListLogLinearPlot[{data1, data2}, Joined -> True, PlotRange -> All];
p=Graphics`Mesh`FindIntersections[pic[[All, 2]]  ]    
(*{{0.841204, 8.84133}}*)  


Show[{pic, Graphics[{Red, PointSize[.025], , Point[p]}]}]

enter image description here

The x value of the intersection evaluates to

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  • $\begingroup$ @Ulrich Neumann It is not working form me, I am using mathematica v11.3. and the result of the previous solution and the solutions you got are different $\endgroup$ – acoustics Oct 18 '19 at 3:19
  • $\begingroup$ @acoustics I don't know your previous result, but the intersectionpoint seems to be ok (see my modified answer). Keep in mind the different coordinates! $\endgroup$ – Ulrich Neumann Oct 18 '19 at 6:36
  • $\begingroup$ I also got the same result but which one I have to consider 2.31916 or 0.841204. $\endgroup$ – acoustics Oct 18 '19 at 6:57
  • $\begingroup$ Probably the last one, if you want the result in the coordinate-system of data1, data2 $\endgroup$ – Ulrich Neumann Oct 18 '19 at 7:00
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    $\begingroup$ Take 2.31916 or alternatively use pic=ListLinePlot[...] in my answer $\endgroup$ – Ulrich Neumann Oct 18 '19 at 7:11

I would generate an InterpolatingFunction for each dataset and then find the intersection with FindRoot:

FindRoot[Interpolation[data1][x] - Interpolation[data2][x], {x, 1}]

FindRoot will look for the root of the given function closest to the provided starting point (in this case: 1).

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  • $\begingroup$ Is it Log value or the normal value, I got an answer which is around 2.304, I am get little confused what value is it. $\endgroup$ – acoustics Oct 17 '19 at 11:27
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    $\begingroup$ The way you plot the data (in this case on a logarithmic chart) does not affect the data itself. So it's the normal value. $\endgroup$ – banone Oct 17 '19 at 11:30

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