A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David C
Date: 2017 Feb 27, 18:59 -0800
Hope this is relevant. Some general info which maybe will help you. All U.S. states, and I imagine all districts & regions, have defined grid coordinate systems for surveyors. Our U.S. system I believe is established and monitored by the U.S. Geological Survey. Here in Massachusetts we have a State Grid. You can go on line and find a “library” of Bench Marks . All BMs are defined via the grid in a coordinate system and latitude and longitude ( I recollect). BMs are round brass discs checked regularly to confirm that they have not been moved or destroyed. Some areas have a large number of BMs, others there can be some distance between them. Sometimes there is a brass disc set in the front steps of the post office. Usually they are easy to spot. The accuracy of BMs vary....First order, second order, etc Some are only x,y. Others are x,y,x. Nav list has a number of surveyors who know far more about this. Maybe you can you can use these to check your IPhone.
What a good idea (-; About 35 minutes walk from here is a trig station. It is popularly known as the Cannon Point Trig although the boring official name is 7503 Upper Hutt No 2.
The GE position is 41° 06.203' S 175° 04.378' E
The LINZ (Land Information NZ ) web sight gives the coordinates of this mark. This is where it gets interesting. The coords are in terms of the NZ Geodetic Datum 2000 and the positions are relative to a tectonic plate. When the plate moves the coords stay the same. Contrast this with wgs84 which is a fixed earth system - when the plate moves the coords change. The two systems were aligned in 2000 and in 2015 the maximum error was 75cm. For the purposes of this exercise I will assume that NZGD2000 and wg284 are identical.
From the LINZ website the position is
41° 06' 12.1390" S = 41° 06.2023 S
175° 04' 22.5954" E = 175° 04.3766' E
The GPS position is 41° 06.203' S 175° 04.376' E
The was some wander but unfortunately I did not have a app that averaged the results (I have since installed one).
The conclusion I draw from this is that in my location GE agrees within a few metres of GPS and the national survey system. That is more than accurate enough for my purposes.