Hi Anand,

Please check the pdf attachments and the links given below.

http://www.tandfonline.com/doi/abs/10.1080/00968205709343501?journalCode=aihq20

https://books.google.co.in/books?id=lvtZTYl7ThgC&pg=PA373&lpg=PA373&dq=methods+to+measure+height+of+stack+for+power+plant&source=bl&ots=Kmx81jwNE2&sig=1ycsOxTMDCQXyYn4jLAO632a7e0&hl=en&sa=X&ved=0ahUKEwiS-YSk9OHMAhUHu48KHcQeBfkQ6AEIKzAC#v=onepage&q=methods%20to%20measure%20height%20of%20stack%20for%20power%20plant&f=false

Good Luck

Please check these links.

http://www.trc.govt.nz/assets/Publications/regional-air-quality-plan/air-plan-index/raqp-appendix8.pdf

http://www.cpcb.nic.in/divisionsofheadoffice/pci2/ThermalpowerPlants.pdf

A basic minimum requirement is. provision of sufficient stack height to prevent downwash during periods of high wind velocity. With stacks less than twice the height of the main powerhouse structure, experience has demonstrated that, during high velocity wind, fumes may be caught in the turbulent vortex sheath.

for thermal power plants approximately 220-275m

You can measure the stack height by using Theodolite or Total station - surveying instruments used in Civil Engineering. These instruments are placed away from stack at ground  and top of stack is sighted and angle of elevation is measured.now considering stack height as h and Tan of angle  of elevation, i.e tan(angle of Elevation)=h/distance from stack to Theodolite measured by tape. Here distance from stack to theodolite by tape is known  and tan of angle of elevation is known, height h can be calculated.Ht of Stack= H=h+Height of instrument(theodolite) from datum. You should take the help from Civil Engg. Deptt. or from civil engineer.