Raw, unedited video taken on July 20, 2018.
OVERVIEW:
I returned to Cape Elizabeth, at Fort Williams Park and Portland Head Light to observe Halfway Rock Lighthouse from two separate elevations.
Halfway Rock Light consists of a light tower and boat house on an exposed piece of ledge. It is just southerly of Webster Rock and a green navigation buoy. According to NOAA chart 13290, entitled "United States - East Coast, Maine, Casco Bay", Halfway Rock Light stands at a height of 76 feet.
EQUIPMENT USED:
Leica GS-16 GPS antenna and CS20 controller with Leica Captivate software.
Leica TS-16 robotic total station (used in manual mode, without controller)
Leica tribrachs
Sokkia heavy duty tripods
Lufkin 6' Engineer's folding ruler
AutoDesk Civil 3D 2018
Apple iPhone 7
PROCEDURE:
I first established my coordinates by GPS readings, via RTN (Smartnet Network), taking 360 epochs on each point. My horizontal values were referenced to the Maine West Coordinate System, North American Datum of 1983 (NAD 83). My elevations were referenced to the North American Vertical Datum of 1988 (NAVD 88).
Point 1 was a kiel dot on a boulder at the water's edge, on a gravel beach approximately 540’ northwesterly of Portland Head Light.
Point 2 was a spike in an overlook on the trail above the gravel beach, approximately 595’ northwesterly of Portland Head Light.
Next, I set my tripods up. I set the TS-16 on tripod at Point 2 and leveled it. While leaving a tribrach at Point 2, I then set up on Point 1 to begin my observation. I recorded the entire procedure via my iPhone 7 in a single, unedited video, presented here. Recording began at 6:02 am, EST.
At each Point, I aimed the TS-16 at Halfway Rock Light, and set my zenith angle to 90 degrees, and observed the tower through the scope.
COLLECTED DATA:
Point 1 (Kiel Dot on Boulder)
Northing: 288250.7636
Easting: 2941441.5303
Elevation: 6.90'
HI: 4.62
Viewing Height: 11.52'
Point 2 (Spike)
Northing: 288250.2486
Easting: 2941371.7617
Elevation: 32.69'
HI: 5.25'
Viewing Height: 37.94'
SOURCED DATA:
Halfway Rock Tower Height: 76'
RESULTS:
I loaded my GPS data into Civil 3D, set up with the Maine West Coordinate Zone, NAD 83 and measured distances to Halfway Rock.
Point 1 to Halfway Rock: Approximately 47,204', or 8.94 miles. (Grid distance)
Point 2 to Halfway Rock: Approximately 47,274', or 8.95 miles. (Grid distance)
Viewing from Point 1, my horizontal crosshairs were intersecting the tower just above the roof of the boat house. The base of the boathouse and the island were not visible.
Curvature calculation:
Distance to Horizon: 4.16 miles
Hidden behind Curvature: 15.26'
Viewing from Point 2, my horizontal crosshairs were intersecting the tower just below the portion of the tower containing the light apparatus.
Curvature calculation:
Distance to Horizon: 7.54 miles
Hidden behind Curvature: 1.32'
CONCLUSIONS:
At a viewing height of just under 38’ in elevation at Point 2, the horizontal crosshairs should have been at the top of the light tower, as in my 2017 observation. (This is easily determined by drawing the Earth to scale and placing the observation height and the tower at their relevant positions.) However, there was likely more refraction during this observation than in 2017, as moisture was visible in the air as the Sun rose in the sky. Refraction would have made the light tower to appear higher than it truly is. Despite any refractory distortion present, the effects of curvature are still clearly observable in this video.
Helpful Links:
NOAA charts are free to download in PDF format and can be found here: [ Ссылка ]
Chart 13290: [ Ссылка ]
Metabunk Curvature Calculator:
[ Ссылка ]
My 2017 observation from the same locale:
[ Ссылка ]
[ Ссылка ]
