Ken,

lat_ts is a setup parameter used in a few of the projections supported by proj (cea, merc, stere, wag3 and wink1) - it is not a magic incantation you can slap on any projection definition string to force it to be "exact" along any given latitude.

But the proj program has functionality to provide a large amount of projection distortion information: Try running

echo 12 55 | proj -VS +proj=merc +ellps=GRS80 +lat_ts=55

Which will give you information about the distortion of a point at 12 E, 55 N (near Copenhagen, Denmark), for a mercator projection with a latitude of true scale of 55. Not surprising, the scale is 1:

#Mercator

# Cyl, Sph&Ell

# lat_ts=

# +proj=merc +ellps=GRS80 +lat_ts=55

#--- following specified but NOT used

# +ellps=WGS84

#Final Earth figure: ellipsoid

# Major axis (a): 6378137.000

# 1/flattening: 298.257222

# squared eccentricity: 0.006694380023

Longitude: 12dE [ 12 ]

Latitude: 55dN [ 55 ]

Easting (x): 767929.55

Northing (y): 4211972.20

Meridian scale (h) : 1.00000000 ( 8.866e-009 % error )

Parallel scale (k) : 1.00000000 ( -1.066e-010 % error )

Areal scale (s): 1.00000000 ( 8.759e-009 % error )

Angular distortion (w): 0.000

Meridian/Parallel angle: 90.00000

Convergence : 0d [ -0.00000000 ]

Max-min (Tissot axis a-b) scale error: 1.00000 1.00000

Then try changing +lat_ts=55 to +lat_ts=54:

#Mercator

# Cyl, Sph&Ell

# lat_ts=

# +proj=merc +ellps=GRS80 +lat_ts=54

#--- following specified but NOT used

# +ellps=WGS84

#Final Earth figure: ellipsoid

# Major axis (a): 6378137.000

# 1/flattening: 298.257222

# squared eccentricity: 0.006694380023

Longitude: 12dE [ 12 ]

Latitude: 55dN [ 55 ]

Easting (x): 786909.29

Northing (y): 4316073.03

Meridian scale (h) : 1.02471546 ( 2.472 % error )

Parallel scale (k) : 1.02471546 ( 2.472 % error )

Areal scale (s): 1.05004178 ( 5.004 % error )

Angular distortion (w): 0.000

Meridian/Parallel angle: 90.00000

Convergence : 0d [ -0.00000000 ]

Max-min (Tissot axis a-b) scale error: 1.02472 1.02472

Indeed a quite useful feature of proj! That said, you will probably also need some basic literature about projections to make real use of this feature.

/thomas