MikePotterfield@GeodeticSolutions.com
Fish,
The dragon moray eel (photo courtesy of Hawaiian Reef
The
Northwest Chain is a string of 11 small island regions running
west-northwesterly from
National Geographical Society map of the Northwest Chain of the
Hawaiian Islands
My
investigations led to the realization that the only people who regularly visit
the Northwest Chain are taking part in scientific research expeditions. So, for a period of three years or so,
whenever I heard of such an expedition being scheduled for this part of the
world, I would enquire with the scientific director to see if a geodesist (me)
could be useful for the upcoming expedition.
Invariably I received polite replies in the negative, but I continued my
enquiries, and eventually I enquired directly with the FWS.
In
the Spring of 2001 Rod Low was senior GIS analyst with
the
If
you want to bring precise geodetic control to remote Pacific islands, a logical
place to look is to the University of Hawaii Pacific
GPS Facility (PGF), whose director is
If
FWS funding had been available for the GPS surveys, PGF probably could have been
engaged to do everything the FWS required, but without funding there was a
limit to what the facility could contribute.
They were prepared to make a substantial contribution, however, not just
out of good will but also to help to upgrade the state’s geodetic
infrastructure. The facility offered to
provide at cost the materials and tools for constructing the permanent poles,
to provide training in the construction of the poles, to provide geodetic GPS
receivers on loan, and to process the baselines resulting from a GPS survey.
And
so on March 3, 2001, I found myself aboard a twin-engined Piper airplane on a
flight from Honolulu to Tern Island in French Frigate Shoals. We flew between
I
was acting as a volunteer for FWS. I
paid for my own airfare between my home in
The
two seats between Tony and me and the cockpit had been removed and were
replaced by my pile of surveying gear.
My company (at that time I worked for Trimble Navigation) had loaned the
expedition a 4700 receiver with L1/L2 microcentered antenna with ground plane,
an automatic level, and a level rod. PGF
provided one of their Z-12 Ashtech receivers with a Dome Margolin antenna, an
80-lb. battery pack to power the receiver’s long baseline observations, the
aluminum antenna pole with associated hardware, an electric drill and other
tools for constructing the pole, and two Wild tripods with tribrachs and
adaptors. My personal gear consisted of
a briefcase with laptop computer, a carry-on canvas bag, and a full-sized
suitcase. Inside the suitcase were my
snorkeling gear and my favorite fish ID book.
My survey gear, minus the aluminum antenna poles
I
also had with me
I
would have to move fast while I was on Tern.
My initial schedule of five days at French Frigate Shoals had been
reduced to three because of a delay in our flight. During these three days I would have to
construct the antenna pole, observe both it and any USC&GS monuments with
GPS, make fast-static imagery registration GPS observations on Tern and its
outlying islands, and carry sea level elevations to the GPS monuments.
When
our plane reached
After
the plane’s departure and a slapdash lunch Tony and I toured the island as part
of my orientation. Rod and Mike had
provisionally agreed on using the roof of the (now empty) generator building as
a location for the permanent antenna pole, and so Tony and I took a close look
at it. (The generator building dates
back to the 1960’s when
Once I went to work, however, things started to go awry. Everywhere I tried to drill holes in the roof, I found myself in layers of tarpaper so deep that the drill bit didn’t even reach the concrete slab. The only way to construct the antenna pole on the roof would have been to hack out a huge hole in the tarpaper, and even then there was no guarantee that the 1-meter pole would project high enough above the roof to make GPS observations.
So
I climbed down to find another site. I
quickly found the ideal spot, a huge, very hard concrete slab over an old
disused rainwater catchment cistern. It
was at least 3 feet high, 25 feet long, and 10 feet wide, and was situated in
an easily accessible spot between the barracks building, where all the
residents live, and the generator building.
There is a clear view of the sky from this slab with the exception of an
old 2.5” diameter flagpole about 30 feet away to the south, which might cause a
minor multipath signal. Otherwise, the
15-degree elevation mask on GPS antennas clears all obstacles. I found Tony in the barracks, and he inspected
and approved the cistern slab as a site for the antenna pole.
The
afternoon sun was starting to drop towards the horizon, and I realized that I
might not be able to complete building the antenna pole before nightfall. If I waited until the pole was constructed
before making GPS observations, I might not be able to make them until the
following day. I decided to pound a PK
nail into the southeast corner of the cistern slab and observe it with the
Ashtech receiver while I built the pole.
In this way, even if I couldn’t complete the pole before nightfall I
could still begin making GPS observations, and I could transfer precise
geodetic coordinates from the nail to the pole after the pole was built. I almost killed myself trying to pound a PK
nail into the extremely hard concrete slab, and all I had to show for my
trouble was 3 PK nails bent into pretzel shapes. I finally came to my senses, climbed back to
the generator building roof and back down again with the drill, and drilled a
small start hole in the cistern slab. I
filled the hole with silicone and pounded a PK nail down into it flush with the
slab’s surface.
I
next pulled out a tripod, tribrach, and adaptor for mounting the Ashtech
antenna over the PK nail. My intention
was to rotate the tribrach inside pencil marks on the tripod plate as a rough
check on the tribrach’s optimcal plummet and fisheye level bubble. But when I went to look into the tribrach’s
optical plummet, it wasn’t there. I
stared at this tribrach in disbelief. I
knew in an abstract way that some tribrachs were indeed manufactured without
optical plummets, but I had never used one myself, and now that was all I
had. Both of the PGF tribrachs were
plummet-less.
Then
I realized, far too late to do me any good, that Eric Kendrick of PGF had
probably tried to warn me about this situation, but I had misunderstood
him. While I had been visiting the lab,
Eric had produced for me an optical collimator, of the type used to adjust
tribrach level bubbles and to calibrate optical plummets. I had thought at the time that this was the
purpose for which Eric had brought out the collimator, but now I realized that
he had probably brought it out so that I could simply center the tribrach over
a survey mark. I should have examined
the tribrachs more closely at UH. I
should have brought the collimator with me.
Worse yet, I had violated the Surveyor’s Cardinal Rule: always wear your
plumb bob! (However, even if I had
brought my plumb bob it still would have been difficult to use it to center the
tribrachs, because the tripods lacked plumb bob attachments for their fastening
knobs.)
I
had to sit down and think. I was
enveloped in a cloud of gloom and despair.
The birds flew and danced and ululated all around me. Quite possibly, I thought, my entire
surveying mission was an abject failure, fouled up beyond repair. It was getting late, and I still had nothing
to show for my efforts.
I
finally determined two methods for centering the tribrachs without optical
plummets, both of which I eventually employed.
The first was to simply place the tribrach down directly over the survey
mark, whenever the tribrach could be seated firmly and securely. This would allow me to position the adaptor’s
bolt within a couple of millimeters of the true center of the tribrach. The second method involved the use of the
automatic level: I could use the vertical crosshair in the level to plumb the
adaptor bolt over the survey mark, but I would need at least two intersecting
lines of sight to position the bolt horizontally, and so this method involved a
tedious series of multiple setups as well as a cooperative and fast-learning
assistant.
I
couldn’t use the first method on the PK nail, because it would have placed the
antenna so low that a pump housing on the cistern slab
would have obstructed the horizon, and moreover the cistern slab surface was so
slick that it didn’t afford a secure seat for the tribrach. So I walked briskly to the barracks to find
an assistant to help me employ the second method. I felt like a complete moron, and I could
only hope that my 4 fellow residents on the island wouldn’t notice my
disorganization..
But
there was no help to be found. The
barracks were deserted, with everyone either behind closed doors or in the
field. Dinner would be prepared at
sunset, and sunset was now close at hand.
I fell
back on my last resort. I returned to
the cistern slab and set up a tripod and the Ashtech antenna on a random,
unmarked, imaginary point. I seated the
tripod legs as firmly as possible on the slab and marked the concrete where the
legs were set, so that I could tell if they moved. I tightened all the screws on the tripod
legs. The generator building shielded
this site from the brisk trade winds, so the wind probably wouldn’t blow over
the tripod, and I could only hope that a red-footed booby or some other bird
wouldn’t crash into it. I turned on the
Ashtech receiver and began my GPS observations.
I called this setup CP Innocence, because it could never be recovered,
and I called the PK nail CP Useless, because in fact it never got used. My only solace was that at least I was doing
GPS, which was the whole point of my being where I was. In the few minutes before dinner I made
repeated trips to the roof of the generator building to bring back down all of
the gear I had hauled up there, including the goshdarn battery pack.
Observing GPS on CP Innocence, with the barracks building in the
background
The
poor beginnings of my survey had put me in a foul mood, but this quickly
cleared away when I joined the genial company at the dinner table. The living is easy on
The
next morning I started to work early, and things began to improve. CP Innocence was undisturbed. I used the 4700 to observe 4 image
registration points at the four corners of the seawall that bracketed the
island’s runway, and I constructed the permanent GPS pole. After building the pole, I mounted the 4700’s
antenna on it and began transferring coordinates from CP Innocence to the pole,
which I named PGF Pole Tern 1. My plan
was to observe three static GPS sessions of at least an hour each in this
configuration: one at the end of the afternoon, one an hour before I retired
for the night, and a third one before breakfast the following morning. The next day I would swap antennas, placing
the Ashtech antenna on Tern 1 and the 4700’s antenna on CP Innocence, and I
would then leave the Ashtech on Tern 1 for the duration of my stay at French
Frigate Shoals.
The 4700’s first session on PGF Pole Tern 1
I
finally mounted the 4700’s antenna on Tern 1 just before six, and began the
first session. Sunset arrived at seven,
and if I moved fast I could squeeze in an hour of snorkeling before
nightfall. I did move fast, and so did
Laura when I came running into the barracks to grab my snorkeling gear. Sarah was in the field, and missed this
opportunity. Laura and I took two of the
Refuge’s bicycles and rode into the face of the tradewinds down the length of
the runway to the eastern end of the island, where a nice little beach (known
locally as
Swimming
north and east from the beach, Laura and I crossed a sandy passageway about
thirty feet deep to the north fringing reef, then followed the reef to the east, then down to the south to a maze
of coral mesas, arches, and caves in about twenty feet of water, and then back
west to East Beach. The coral around
Tern isn’t as various or colorful as it is in the more southerly latitudes of
the main
We
exited the sea in the day’s last light and cycled back to the barracks, this
time coasting along with the tradewinds at our backs, as the orange disk of the
sun sank down on the clear horizon over the west end of the runway. Laysan albatrosses soared past our ears no
more than three feet from our heads. As
we approached the barracks the sun IKONOS near-infrared
image of
My
own mood was greatly enhanced by the suceess of my GPS observations. By dinner time I had already observed more
than an hour of static GPS with the 4700 on the new pole, and I had accumulated
more than 24 hours observing CP Innocence with the Ashtech. Even if the tripod on CP Innocence were to be
knocked over, I should nevertheless be able to transfer precise coordinates
from this unmarked point to the permanent antenna pole. My strategy with CP Innocence had been
vindicated, and by breakfast the following morning I would have even better
observations on these two setups.
Back
at the barracks, just before dinner, I turned off the 4700. Later that evening, an hour before retiring,
I took a flashlight and, stepping gingerly through the crowd of gray albatross
chicks roosting in their little depressions in the ground everywhere, and
trying to avoid stepping in noddy burrows, I returned to the cistern slab and
turned on the 4700 receiver for another 1-hour session. There was a bright three-quarter moon, and I
took the opportunity to watch the birds swarming and screaming in the sky, the
Laysan albatrosses continuing their odd dances and bill-clacking, and the bold,
curious little white terns hovering just over my head.
The
following day, March 5, was a sunny, clear day for my trip to
The NOAA nautical chart for French Frigate Shoals
Our
first destination was
Approaching
We
landed on
While
I was hauling gear up the beach I got slammed in the head out of nowhere. I looked around to see a big black frigate
bird soaring away. I touched my forehead
and found a small smear of blood. Looking
up the beach, I saw that I had parked my gear next to a bush that had a frigate
bird’s nest in it. I moved my gear.
My
next registration spot was at the tip of vegetation on the northwest end of
Our
beach landing on Little Gin Island was more challenging, and was in fact the
most exciting part of my trip to the outer islands of French Frigate Shoals. Heavy swells were hitting the beach on both
sides, and at first it looked like we wouldn’t be able to land. But some careful maneuvering by Tony, through
the reef on the crest of the swells, got us close to the beach near the
southwest tip of the island, and I jumped off the bow into the water. The swells were so large that the troughs of
the swells were at my waist, but the crests of the swells were over my
head. I held my breath and raised my
arms and, while underwater, I felt the receiver get dumped into my hands. I managed to walk up the slope of the beach,
holding my breath, until I was on dry land.
I found the usual crowd of birds on the crest of the tiny island, but
nothing else except sand.
The
sea conditions looked marginal. A squall
was moving in from the northeast. I
decided to observe my registration point as close to the boat as possible, on
the south tip of the island, so that if necessary I could race all the gear
back to the boat before sea conditions forced the red whaler to leave its
anchorage.
My
last image registration point on the outer islands was on
The Ashtech is moved to its last occupation, PGF Pole Tern 1
Tony
and I arrived back at
The following day, March 6, was the day the Townsend
Cromwell, a 165-foot NOAA research vessel, would carry me away from French
Frigate Shoals to
I
had wanted to find at least three USC&GS monuments, for the best possible
transformation of the astronomical datum, but this one (RM2) would have to
do. I lovingly wiped the guano off the
disk and admired the familiar stamping on it.
I tried to place the tripod over the pillar, but the legs were too short
for a setup, so I employed my first method of placing the tribrach directly
down on the station mark, where a T3 astronomical theodolite had originally
been mounted.
Red-footed boobies nesting between FRIG RM 2 1961 and the old fuel shed
While
the 4700 observed the brass disk I enlisted Sarah to help me carry elevations
from the tide gauge on the west end of the island down to PGF Pole Tern 1 and
FRIG RM 2. We recovered three epoxied
bolts recorded as bench marks for this station, and we ran a level loop that
closed with an error of 6 mm. The UH Sea
Level Center provided a value for mean sea level at the tide gauge for the
period from 1974 to the present, and this value can provide an initial estimate
of the geoid separation at Tern Island.
Differencing the astronomical coordinates at FRIG RM2 against the
geodetic coordinates will provide an estimate of the slope of the geoid, in the
form of deflections of the vertical in latitude and longitude.
The
At
After
lunch I packed up all of my gear, and at
Aboard the Cromwell
The
Cromwell was to be my home until my return to
NOAA photograph of the Townsend Cromwell
In
the wet lab aboard the Cromwell were the materials for constructing another PGF
antenna pole, this time on Laysan.
Included in these materials were 750
Boarding the Townsend Cromwell for Laysan from Tern, LaPerouse Pinnacle
watching
The
food was good and the living was easy aboard the Cromwell during our two-day
passage to
Arrival at
Unfortunately,
however, seas as high as 30 feet prevented making a beach landing when we
arrived. The entire beach was closed off
by surf. The Cromwell altered its agenda
by leaving Laysan for
We
returned to Laysan, but the seas continued to prevent a beach landing. For two days the Cromwell circled Laysan,
waiting for a chance to offload. The
only way that the Cromwell could have returned to
All
visitors to
The
FWS and NMFS camps are close at hand to the small cove on the northwest corner
of the island that is used for boat transfers.
My guide was Eric Lund, a FWS volunteer who would be returning to
I
would have preferred to use RM2 for my long-baseline occupation, but a wooden
antenna mast for a defunct radar had been lashed to the concrete pillar, and
neither GPS antenna could be positioned directly over the brass disk because of
it. Instead, I used LAYS 1961, a brass
disk in a concrete post projecting about a foot above the sand, as the
long-term occupation. I plumbed the
antenna over the station mark with Eric’s assistance using the auto-level
method.
Looking southeast to LAYS 1961 and the FWS camp, with Laysan’s lake in
the background
Somehow
I would have to transfer coordinates to LAYS RM2, but I didn’t have any time to
waste. I made two arbitrary black marks on
the westerly top of RM2’s pillar, called them RM2-A and RM2-B, and carefully
measured the short distances between them and the punch mark on the brass
disk. The coordinates of LAYS RM2 1961
could be accurately computed using these observations. I then occupied each of these eccentric
stations for about a half-hour each, while Eric and I chose image registration
points, planned our expedition around the island, and ate a lunch of power
bars.
The
FWS camp would show up well on the IKONOS image of Laysan, and so the pillar
could be used as a registration point.
Eric and I chose and old shipwreck on the south end of the island, and
an unusual formation of rock on the east side of the island, as the other two
registration points, and we began our hike down the western edge of the island
to reach them.
It took about an hour. Wildlife regulations required us to make wide detours around the monk seals on the beach, and if a misstep should collapse a bird burrow we had to dig it out. Albatross chicks, frigate bird nests, red-tailed tropic birds, and boobies were everywhere, and it was impossible to walk in a straight line. Laysan’s westerly beach is sandy and lovely, and just off the beach the coral reefs and brilliant turquoise water promised superb snorkeling. I enjoyed the beauty of the scene even more than I felt sorry for myself for not being able to go snorkeling there. Finally we reached the shipwreck at the tip of the island, and observed the north end of the old steel ship’s prow sticking up through the beach sand.
The LAYS RM2 1961 pillar, with the radar antenna pole lashed to the
east side
As soon as this session was complete we resumed our hike, proceeding north along the island’s easterly shore, until we reached the rock pile at the edge of the vegetation line. I deployed the receiver’s antenna in the middle of this rock pile for our second registration point.
We
were moving as quickly as we could. I
wanted to observe another USC&GS monument known to FWS personnel to be on
the northwest end of the island, and to reach it Eric and I moved inland, to
the east shore of the unique brackish lake in Laysan’s interior. By moving along the fringe of the lake we
could make the best time. A grove of
coconut palms is situated at the north end of the lake, and by hiking through
this grove and up to a low ridge we could reach the monument. I think this monument is probably AZ. MK., an
unstamped disk, but I wouldn’t know for sure until I saw it. The 1961 documentation also records LAYSAN
MAGNETIC somewhere in the northeasterly area of Laysan, but because there is no
other record of this monument and because there is no recorded distance to it
from LAYS and because the FWS personnel knew nothing about it, I didn’t even
look for it. There wasn’t enough time.
Throughout
this trek I had been carrying a hand-held radio, with which I monitored
communications between the Cromwell and the shore parties. Sometimes our reception was interrupted by
line-of-sight obstacles, but we always moved back into range after a few
minutes. Eric and I were on the easterly
shore of the lake, opposite the FWS camp, and moving quickly towards the
coconut grove, when I heard the captain of the Cromwell on the radio summon a
search party to find us. His
instructions were to send one of the Cromwell’s boats to circumnavigate the
island, to see if we could be found.
Obviously he had been trying to communicate with me while I was out of
radio range.
I
called the Cromwell and canceled the search party. Captain Callahan sounded relieved to hear
from me, but he had bad news. The
offload of goods and personnel from the Cromwell to the beach was complete, and
so was the onload of goods from the beach to the Cromwell. Everyone still supposed to board the Cromwell
was on the beach, ready to go, except for my guide and me. I was instructed to return to camp, dismantle
my GPS equipment, and report to the beach immediately.
Eric
and I walked briskly around (and, in places, through) the lake, and up a slight
slope to the FWS camp. There I turned
off the Ashtech receiver, after logging a session of 5.5 hours. It will take some good luck, low ionospheric
energy levels, and expert baseline processing to produce good baselines from
LAYS 1961 to the CORS sites.
Everyone
was eager to help me pack up my gear and carry it down to the beach. We waded into the surf, loaded the gear into
the boat, and as soon as I climbed aboard the coxwain gunned the motor and
ferried us back to the Cromwell. I had
been on Laysan for a total of 7 hours.
The Cromwell along Kauai’s NaPali coast on her return to
The
Cromwell sailed back to
Both
GPS surveys were post-processed by WAVE using the precise IGS ephemeris and
holding fixed ITRF97 coordinates on the CORS stations
|
|
FRIG RM2 1961 |
LAYS RM2 1961 |
LAYS 1961 |
|
Geodetic f |
N 23°52'10.475108" |
N 25°46'27.362395" |
N 25°46'28.026712" |
|
Geodetic l |
W 166°16'58.852292" |
W 171°44'20.136409" |
W 171°44'20.064546" |
|
Astronomical f (1961) |
N 23°52'21.440000" |
N 25°46'24.890000" |
N 25°46'25.550000" |
|
Astronomical l (1961) |
W 166°17'03.730000" |
W 171°44'27.990000" |
W 171°44'27.910000" |
|
H |
19.7865m |
19.2100m |
17.8971m |
|
h (sea level) |
2.7210m |
------------------- |
--------------------- |
|
D f |
-10.964892” |
2.472395” |
2.476712” |
|
D l |
4.87771” |
7.85359” |
7.84545” |
|
N (observed) |
17.0655m |
-------------------- |
--------------------- |
|
x (observed) |
10.964892” |
-2.472395” |
-2.476712” |
|
h (observed) |
-4.46052” |
-7.0722749” |
-7.064928” |
|
|
1.9740” |
3.414938” |
3.411433” |
|
N (from geoid model) |
15.1111m |
7.2014m |
7.2007m |
|
x (from geoid model) |
0.86456” |
-7.001143” |
-7.001143” |
|
h (from geoid model) |
-3.0365” |
-.674589” |
-.674589” |
|
LaPlace Corr (from
model) |
1.34367” |
.749107” |
.749107” |
There
are significant differences between the observed deflections of the vertical
for both surveys and the deflections of the vertical estimated from the geoid
model. The reason for the difference is
graphically illustrated by a wire frame 3D image of the WW15MGH geoid as it is
modeled from the main
A 3D representation of the geoid (WW15MGH) along the
Hawaiian Ridge
The
geoidal slopes along the Hawaiian Ridge are so steep and deep that a relatively
small change in horizontal position will result in dramatically large changes
in geoid slope and geoid height.
Another
(profile) view of the geoid along the Hawaiian Ridge clearly reflects the
remnant islands that constitute the Northwest Chain.
The profile of the geoid
surface running along the Hawaiian Ridge.
It
will obviously require a great many well-distributed observations of geoid
height to define the geoid accurately along the Hawaiian Ridge.
Although I began this project without much fanfare, by the time I actually made the trip a great deal of new interest in the NW Chain had been generated by the Coral Reef Ecosystem Initiative announced by President Clinton in his last month in office. In August and September NGS mounted two expeditions to the NW Chain to bring in GPS to all of the islands. Information about these expeditions can be found on the NGS website, www.ngs.noaa.gov.
Also,
since this report was written, UHPGF has produced a Pacific GAMIT solution for
the primary monuments on Tern and
Information
about working as a volunteer with the USFWS can be found on this agency’s web
page at www.fws.gov. Any questions about this article can be
directed to me at mikepotterfield@geodeticsolutions.com
and by visiting www.geodeticsolutions.com.. Rod Low can be
contacted at the ESRI offices in
I
owe a debt of gratitude to everyone I met and worked with during my expedition
to the Northwest Chain of the