To measure the area of an arbitrary two dimensional shape you will use a planimeter. To measure an area you could also just use a simple ruler depending on how big the area is.
1/2(bh)
Pi (3.141...) is used for circles, and circles only. You use it to find the area and circumfrence.
-- Find out the population of the area. -- Find out the area of the area. -- Divide the population by the area. The result is the population density of the area.
Pi is used to find the area and perimeter of a circle and the volume of a sphere, cylinder, and cone. Pi=3.14159265358979323846264338327950288419716939937510582097494459230781640628620899862803482534211706798214808651328230664709384460955058223172535940812848111745028410270193852110555964462294895493038196... That was 200 decimal places of Pi.
ruler
the instrument that the used was an intelligent brain
The best place to find used drums sets may be online, or at a used instrument store. You can visit one in your area and perhaps get used drums sets for a cheap price.
A compass is used to find your location in a forest
A violin can be used, but its effectiveness is limited.
A Planimeter.
Graduated cylinder
Pressure gauges.
A scale
This instrument is called pluviometer (a rain gauge). See the link below.
A set of scales
radar
Scales.