I am working as a structural engineer for a very old processing plant. One of my main responsibilities is investigation of existing structures either to repair damage accruing from materials having reached their safe life expectancy, or retrofit to support new equipment installation.

There are many parts of this plant that are difficult to access, and very hard to move around in easily. Often I find I require reasonably accurate measurements (accurate enough for repair construction documentation) between points that are view-obstructed. Even with an assistant often this is just not doable, yet it is nevertheless essential.

What I'm looking for is some sort of easily portable device that I can use to establish differences in spatial location between points - basically plane coordinates plus elevation. Ideally this would be some sort of GPS device similar to what Trimble makes (or used to make) but not requiring the same sort of accuracy needed in cadastral surveying, e.g. Within an eighth-of-an-inch would be satisfactory. Perhaps it might be a handheld unit that "reports" to a stationary unit for relative measurement.

The important thing is that said system be portable, and durable! This is not exactly a pristine environment, not to mention dry and temperature-controlled!

Recommendations are most heartily welcome - and don't be bashful if you're a sales rep of some kind. I need information.

Thank you.

"No one is completely useless. He can always serve as a bad example." --My Dad ca. 1975