EllipseFit is an integrated program for geological finite strain analysis. It includes routines to derive three-dimensional strain from oriented digital photographs, designed for field-based structural geology studies. The intuitive graphical interface and multi-platform deployment also make it ideal for introductory structural geology laboratories.

Images of oriented sections can be digitized using center-point, five-point ellipse, or n-point polygon moment-equivalent ellipse fitting. The latter allows strain calculation from irregular polygons. Graphical strain analysis techniques include center-to-center methods, with manual and automatic n-point ellipse-fitting.

Graphical displays include axial length graphs, Rf/Φ graphs, logarithmic (Elliott plot) and polar graphs with automatic contouring, and strain maps.

Best-fit ellipse calculations include shape-matrix eigenvalue, mean radial length, and hyperboloidal vector mean. Shape-matrix error analysis is done analytically and using bootstrap techniques. The initial data set can be unstrained to check variation in the calculated pre-strain fabric.

Fitting of ellipse-section data to a best-fit ellipsoid (b*) is done using the shape-matrix technique of Shan. Error analysis is done by calculating the section ellipses of b*, and comparing the misfits between calculated and observed section ellipses.

Graphical displays of ellipsoid data include axial-ratio (Flinn plot) and octahedral strain magnitude (Hossack plot) graphs. Data can be directly digitized from photographs, or can be imported using comma or tab delimited spreadsheet files. Graphs can be exported as bitmap images in various formats, or as vector graphics files which can be imported into graphics editing programs, such as Adobe Illustrator.

EllipseFit is written by Frederick W. Vollmer and is freeware. Note that EllipseFit 3 is a major upgrade that is still in development stage (beta). It provides some new techniques, including line stretch analysis and RMS error calculations, but others are not yet implemented. Please read the accompanying ReadMe files for information on license, citation and references.