what is a merkland and other old measurements

Here is an extensive discussion of old land measurements :

Land was measured in Value - not Area

As silver was weighed by troy weight, in which there are 20 dwt. to the ounce, an unciate, or davoch, of land was divided into 20 penny lands, each of which was worth, in the first valuations, 2s 8d Scots, 2|d sterling.
It must be clearly understood that a penny land is not land worth a penny, but land worth the twentieth part of the sum an unciate is worth at any given time.
In some charters, in which smaller areas of land are granted, two other terms are used, the "pound land," which was worth £1 Scots, or 1s 8d sterling, and the merkland, which was worth 13s 4d Scots, or 1s 1 1/3d sterling.

In Skye the valuation of farms is given in penny lands.
In Islay the unit was the merkland, which contained five penny lands.
For purposes of comparison I shall in all my calculations employ the merkland as the unit, dividing the unciates by 4, and multiplying the penny lands by five.