Finding an inexact result
I have a simple table with 4 fields. Species; Age; Height; and Yield Class
I want to find the yield class, for a given Species Age and Height.
I have it working if exact numbers are put in for the three fields. Species and age are definitive, but the height I find in the forest may not be represented in the database. For example I may measure a tree and record 21.7m. However in the database 21.7 may not exist - the lower figure may be 20.5 (with a Yield Class of 6) and the next higher figure may be 22.0 (with a Yield Class of 8) In this case the query does not give a result, but I need it to return the lower Yield Class (6 in this case).
Can any one advise how I can get my inexact result, please?
I suspect it's simple, but then I'm a simple guy.