Computers make our calculations easier to do, but what about our input?

The basis of a DraughtSurvey is finding the draughts. This can be by reading or by measuring.

But that's not all that's needed!

We need:

• density of the water in which the vessel floats
• the amount of liquids on board (not the beverages)
• other items that will be loaded or discharged while the cargo is being transferred.

Several entities (companies, governmental bodies etc.) have published studies regarding the accuracy of a DraughtSurvey. Some even intended to establish a better method to replace the (UN/ECE-based) DraughtSurvey. Although I cannot find the relevant info on-line anymore, I know some study was going on in the USA.
Of course it goes without saying that every surveyor should have his own notebook and write down all observations. When there are more parties involved, readings should be agreed upon. It is however known that some tend to 'read' in their favour.

Finally, when all needed information is obtained, the calculations can begin.

First we apply the corrections on the obtained draughts, to have the draughts on the perpendiculars. Now we have the trim, list and hogging or sagging.

Next step is to calculate the mean (Mean of Mean of Means) draught.
And with this number we enter the hydrostatic particulars (maybe even after applying a keel correction). Here we look up the data (bookdraft, displacement, tpc, lcf, mtc) we need for our interpolations and calculations. We calculate the corresponding displacement in sea-water for this draught. Because the ship has trim, we also have to apply corrections for this to reach a displacement in sea-water for this trim (and draught).

Next step is the correction for density. Seldom we have have harbour-water with a density of 1025 kg/m3.

After we've finished the calculations we have found the (famous) 'constant'. It is called a constant as this unknown weight does not change during the cargo-operations.
Normally it also doesn't change (much) during the vessel's life and for this many take a holy approach to this number

Draught - reading or measuring or . . .

According to the general perception of a DraughtSurvey, the draughts should (or must) be read.

Sometimes this is impractical, the ship may be to close to the quay, there are no draught-marks, the water is rough etc. etc.

Before we start obtaining the drafts, we should familiarize ourselves with the marks on the hull, metric or imperial (metres or feet+inches). Also we should try to verify the size of the marks, be these painted or welded.
When it is impossible or unpractical to read the draughtmarks, the draught should be measured from a (fixed) point of reference such as a deck-line. It is recommended to use this point of reference during initial and final survey to avoid errors.
When measuring, a calibrated device must be used.

It's not all cargo that's inside a ship.

For various reasons a ship carries ballast. Most of the time it is to have enough draught for a propeller immersion and steering power. Another reason can be it's stability.
In general when cargo enters a ship, ballast will be taken out.
As it has weight, it has to taken into consideration and included in the calculations. Therefore all ballast-tanks have to be sounded.

There are several requirements for this action:

• no other actions may take place that change the position (trim and list) of the ship
• no manipulation of ballast
• total height of sounding pipe
• level of ballast-water in each tank
• density of ballast-water (each tank)

Here we have to use various aids, such as a calibrated sounding-tape, water-finding paste, hydrometer.
Before we even can start calculating, we must know the trim of the vessel and her list (heel).
The ship must have a sounding-table for each tank, preferably with corrections for trim and list. Only when we have obtained this information, we can proceed to our calculation.
We look up in the corresponding table for each tank the data we need for the trim and heel/list correction (interpolation!) and then the corresponding volume can be calculated. This volume is then multiplied with the density and we have the weight.

At least the chief-officer will accompany the surveyor . . .

As transport is business and contracts are based upon the weight of the goods, buyer as well as seller will send their representative for the DraughtSurvey. To avoid any bias, an impartial or independent surveyor can be present as well.
Here the game gets interesting.
Whether we like it or not, we are all biased, to defend the interests of our clients. Buyer's and seller's surveyors have obvious interests, but what about the independent surveyor? He is nominated by the seller, with approval of the buyer. He must walk the line to keep both parties happy. But also himself, if the vessel discharges short, he might lose the confidence of the buyer. So the role of the independent surveyor is a very important one.
His role should be well defined in the contract between buyer and seller.