FITCIRCLE(1)

NAME
fitcircle - find mean position and pole of best-fit great [or small] circle to points on a sphere.

SYNOPSIS
fitcircle [ xyfile ] -Lnorm [ -H[i][nrec] ] [ -S[lat] ] [ -V ] [ -:[i|o] ] [ -bi[s|S|d|D[ncol]|c[var1/...]] ] [ -f[i|o]colinfo ]

DESCRIPTION
fitcircle reads lon,lat [or lat,lon] values from the first two columns on standard input [or xyfile]. These are converted to Cartesian three-vectors on the unit sphere. Then two locations are found: the mean of the input positions, and the pole to the great circle which best fits the input positions. The user may choose one or both of two possible solutions to this problem. The first is called -L1 and the second is called -L2. When the data are closely grouped along a great circle both solutions are similar. If the data have large dispersion, the pole to the great circle will be less well determined than the mean. Compare both solutions as a qualitative check.

The -L1 solution is so called because it approximates the minimization of the sum of absolute values of cosines of angular distances. This solution finds the mean position as the Fisher average of the data, and the pole position as the Fisher average of the cross-products between the mean and the data. Averaging cross-products gives weight to points in proportion to their distance from the mean, analogous to the "leverage" of distant points in linear regression in the plane.

The -L2 solution is so called because it approximates the minimization of the sum of squares of cosines of angular distances. It creates a 3 by 3 matrix of sums of squares of components of the data vectors. The eigenvectors of this matrix give the mean and pole locations. This method may be more subject to roundoff errors when there are thousands of data. The pole is given by the eigenvector corresponding to the smallest eigenvalue; it is the least-well represented factor in the data and is not easily estimated by either method.

-L  Specify the desired norm as 1 or 2, or use -L or -L3 to see both solutions.

OPTIONS

xyfile
    ASCII [or binary, see -b] file containing lon,lat [lat,lon] values in the first 2 columns. If no file is specified, fitcircle will read from standard input.

-H  Input file(s) has header record(s). If used, the default number of header records is N_HEADER_RECS. Use -Hi if only input data should have header records [Default will write out header records if the input data have them]. Blank lines and lines starting with # are always skipped.

-S  Attempt to fit a small circle instead of a great circle. The pole will be constrained to lie on the great circle connecting the pole of the best-fit great circle and the mean location of the data. Optionally append the desired fixed latitude of the small circle [Default will determine the latitude].

-V  Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].

-:  Toggles between (longitude,latitude) and (latitude,longitude) input and/or output. [Default is (longitude,latitude)]. Append i to select input only or o to select output only. [Default affects both].

-bi Selects binary input. Append s for single precision [Default is d (double)]. Uppercase S or D will force byte-swapping. Optionally, append ncol, the number of columns in your binary input file if it exceeds the columns needed by the program. Or append c if the input file is netCDF. Optionally, append var1/var2/... to specify the variables to be read. [Default is 2 input columns].

-f  Special formatting of input and/or output columns (time or geographical data). Specify i or o to make this apply only to input or output [Default applies to both]. Give one or more columns (or column ranges) separated by commas. Append T (absolute calendar time), t (relative time in chosen TIME_UNIT since TIME_EPOCH), x (longitude), y (latitude), or f (floating point) to each column or column range item. Shorthand -f[i|o]g means -f[i|o]0x,1y (geographic coordinates).

ASCII FORMAT PRECISION
The ASCII output formats of numerical data are controlled by parameters in your .gmtdefaults4 file. Longitude and latitude are formatted according to OUTPUT_DEGREE_FORMAT, whereas other values are formatted according to D_FORMAT. Be aware that the format in effect can lead to loss of precision in the output, which can lead to various problems downstream. If you find the output is not written with enough precision, consider switching to binary output (-bo if available) or specify more decimals using the D_FORMAT setting.

EXAMPLES
Suppose you have lon,lat,grav data along a twisty ship track in the file ship.xyg. You want to project this data onto a great circle and resample it in distance, in order to filter it or check its spectrum. Do the following:

fitcircle ship.xyg -L 2

project ship.xyg -Cox/oy -Tpx/py -S -F pz | sample1d -S-100 -I 1 > output.pg

Here, ox/oy is the lon/lat of the mean from fitcircle, and px/py is the lon/lat of the pole. The file output.pg has distance, gravity data sampled every 1 km along the great circle which best fits ship.xyg

SEE ALSO
GMT(1), project(1), sample1d(1)