Maps can be used to give an easily-interpreted summary of observations on ecological variables, or on plant or animal distributions. The contour or isopleth intervals used should be those which account for most of the variation in the original data. The most common interval-selection methods, the use of quartiles or equally-spaced intervals, in many cases do not meet this criterion. The use of clinographic curves, which often give better intervals than the other methods, is discussed in relation to the effects of soil factors on the distribution of Eisenia rosea (Savigny). A correlation coefficient for evaluating the interval-selection methods is given.