Recently, one of my construction friends referred to me regarding the datum in cadastral.
According to JUPEM circular KPUP 6/1981, there are 3 types of datum.
First datum is 2 (two) old marks found in position where the difference in distance is 50mm in 40 meter length. These both old marks can be observed or computed cum one solar observation to prove the bearing. If the difference in bearing between solar and old is nearly perfect (10” – 30”) then there is your datum.
Second datum is 3 (three) old marks in position. The angle for first class survey is 10” and the second class is 30”. The distance can be directly observed or computed. For the case of computed, we need a solar observation to prove your bearing.
Third datum is by GPS but sorry I still do not know how to apply.
According to my friend, road & drain are from Engineer plan while building plan is from architect. When he superimposed these both digital data and noticed that both plans can not be matched. After thoroughly checked and found that the architect did not follow the actual boundary dimension. This is normal scenario and architect does not bother the boundary issue. They ensure that their structure design is within the boundary and the less is other professional business. Thus, land surveyor must justify ourselves before start set the structure according to their plan. The best solution is preparing a precomp plan and endorse by a Project Manager referring to architect plan.
Setting out in construction project where I can relate to fitting concept. In this case, each professional (land surveyor, engineer & architect) have own fitting. So, orientation plays an important role in apply for the setting out. Baseline is a main keyword for the whole concept.
Basically, baseline is a concept of comparison of new survey with old survey and land surveyor need to justify ourselves whether the marks are in position or otherwise. Land surveyor can not start from datum marks and make a conclusion that the others marks are out by coordinate method. Present cadastral survey in Malaysia is involving of bearing and distance unless JUPEM start using least square adjustment method in future. All the lots in the certified plan or PA must have mis-close and seldom have perfect mis-close unless the lot is small. This is the main reason why coordinate system can not apply. If the whole lot have mis-close of 0.3 m then one of the marks in coordinate will have 2 coordinates by difference in 0.3 m.
Once the land surveyor finds the perfect baseline (3 marks in position) then apply the orientation. Then coordinate system only can start apply in setting out and locate other old marks either in position or no.
Even though, presently digital data are available from the entire professional in cad format but land surveyor must be careful when using the data. Insertion of common point is important and we can not just insert where the datum started and will cause the whole structures or engineer/architect boundary also will shift into a direction.
The conclusion is look for a baseline before using coordinate system to set your structure in construction.