I have a top of the line (50 years ago) Heath "Hezzanth" metal sextant and a Davis plastic sextant. I can confirm Maurice's assessment that neither compares with the precision obtained by the cheapest GPS. The error attributed to the dimensional changes in the plastic sextant by temperature and humidity variations is insignificant when compared to the position finding error contributed by me. With my GPS I can find my position to about 10 feet. The error in my charts is a good deal greater than that. With either sextant I am lucky if I can place myself within ten miles. And I know how to use a sextant. I used to teach celestial navigation to NROTC cadets. I currently keep the plastic sextant and a Pulsar railroad approved watch as a chronometer substitute, in my emergency navigation box. I hope that I never have to use it in earnest when approaching an unknown rocky shoreline. Incidentally, with the sextant, the watch and a calender you can make a reasonable estimate of position by sun sights alone. No almanac needed. That is if your criterion of "reasonable" is about 100 miles. The polished up Heath "Hezzanth" sextant is now a fine mantel display. Larry Z