I have had lots of interest and questions on this topic. Well obviously it takes a long time to become proficient in estimating the volume of timber on a tract of land and I can not cover the topic in depth on this blog. However it is worth talking about so you, the landowner has a general knowledge about the topic. Cruising timber is actually a statistical exercise. The forester must determine how many trees to actually count and measure to estimate the amount of timber on a given tract of land. The practitioner walks through the woods and takes sample plot at predetermined spots on the land. For the sake of simplicity we will only discuss fixed area plots here i.e. if the forester estimates how much timber is on that 1/10 acre plot, he simply mulitplies by 10 to estimate the total volume on that acre of land. Likewise this approach can apply to a hundred acre tract or a thousand acre tract.
Next, the trees he measures are converted to volume. Volume can be estimated in MBF (thousand board feet), tons or cords. The following link show the various volumes for comparison sake.
This is a very simplistic approach to a complicated exercise, if you have specific questions, please comment.