In my power company days, we hired a professional surveyor. There are a couple of issues to contend with in transmission (and distribution) line design. You have to measure clearances over roads, the ground, and from other structures to do proper line design for conductor sag and swing under various loading conditions. This was something I often did myself. But the other function of a survey is to locate right-of-way boundaries, property corners, easements, etc. There are legal implications involved with this in the event that your line ends up encroaching on someone's property. The liability to your company needs to be considered in the event an error is made. One pole set on private property that must subsequently be relocated is a lot less expensive than an entire transmission line. Also, there are regulations regarding the recording of surveys. When and how they must be recorded, professional surveyor licensing requirements, etc. if easements or permits must be obtained.
Each jurisdiction is different, so you need to find the regulations applicable in your area.
