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How to Digitize Logarithmic & Polar Plots in Engauge Digitizer

⚡ Quick Answer

In the Axis Wizard, set each axis scale to Logarithmic instead of Linear — X and Y are independent, so semi-log and log-log plots both work. Place your three axis points exactly on labeled grid lines (e.g. 100, 101) and type the matching values. For polar plots, choose Polar coordinates in the same wizard.

Digitizing Semi-Log and Log-Log Plots

  1. Import: Drag your chart image into Engauge. The Axis Wizard opens automatically.
  2. Choose scale: For each axis, change the Scale dropdown from Linear to Logarithmic. Leave an axis on Linear for a semi-log plot; set both to Logarithmic for log-log.
  3. Place axis points: Click three points on known grid lines — for example the lines labeled 1, 10, and 100 on a log axis.
  4. Type exact values: Enter the true values (1, 10, 100). On log axes, being off by one decade multiplies every result by 10, so double-check.
  5. Trace: Use Segment Fill or Point Match to trace the curve, then export. Engauge converts the logarithmic positions back to real numbers automatically.

💡 Tip: Scan or export your chart at the highest resolution available. On log axes, a 2-pixel placement error near the bottom decade can mean a large relative error in the extracted value.

Digitizing Polar Plots

  1. Select Polar: In the Axis Wizard, choose Polar as the coordinate system.
  2. Define center: Click the exact center of the polar chart.
  3. Define radius: Click a point on a labeled radius circle (e.g. r = 5) and type its value.
  4. Define angle: Click a point on a known angle line (e.g. 90°) and type its value.
  5. Trace and export: Trace the curve; exported data comes out as radius/angle pairs.

Dealing with Grid Line Interference

Logarithmic charts often have dense minor grid lines that confuse automatic tracing. Three defenses:

  • Color filtering: if your data curve is a different color from the grid, filter the image to that color before tracing — this is the single most effective trick.
  • Discretize carefully: the Discretize filter samples along the curve; on log axes use a fine step so decades with steep slopes are captured accurately.
  • Manual tracing with zoom: for publication-quality data, trace critical regions by hand using the zoom window. It is slower but the most accurate option.

Frequently Asked Questions

Can Engauge Digitizer handle log-log plots (both axes logarithmic)?

Yes. In the Axis Wizard you set each axis independently, so you can choose Logarithmic for the X axis, the Y axis, or both — a log-log plot is simply both set to Logarithmic.

My logarithmic axis values look wrong after digitizing. What went wrong?

Almost always, an axis point was placed on the wrong decade — for example clicking the “10” grid line but typing “1”. Double-check that each axis point’s typed value matches the grid line you clicked, paying attention to powers of ten.

Can I digitize polar plots (radar charts)?

Yes. Choose Polar coordinates in the Axis Wizard instead of Cartesian. You then define the center point, a known radius value, and a known angle — Engauge converts the traced points to radius/angle pairs.

Grid lines keep getting traced as data. How do I stop that?

Use color filtering before auto-tracing: if your curve is blue and the grid is gray, filter for blue only. You can also erase grid lines with an image editor before importing, or trace manually with the zoom window for full control.

Ready to try it yourself?

Download Engauge Digitizer free and follow along with this guide.

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