For each axis, the LabelAt value gives the value on the other axis at which numerical labels and tick marks should be placed (remember that Plots suitable for use with astGrid may only have two axes). For example, in a celestial (RA,Dec) coordinate system, LabelAt(1) gives a Dec value which defines a line (of constant Dec) along which the numerical RA labels and their associated tick marks will be drawn. Similarly, LabelAt(2) gives the RA value at which the Dec labels and ticks will be drawn.
The default bahaviour is for the Plot to generate its own position for numerical labels and tick marks.
AST A Library for Handling World Coordinate Systems in Astronomy