Problems With The Vertical Reference Frame In Alaska

1
Problems With The Vertical Reference Frame In
Alaska

• JOA Surveys
• Erik Oppegard

2
Two Problems for Today

• Distribution and density of vertical control
• Time

3
Where Is this?

• State of Wisconsin
• 295 miles E-W
• 320 miles N-S
• 11,090 NAVD88 published stations
• Average 1 NAVD88 benchmark every square 8.5mi

4
Where Is This?

• State of Alaska
• 800 miles E-W (Canada border to Hooper Bay)
• 800 miles N-S (Barrow to Seward)
• 3608 NAVD88 published stations
• Average 1 NAVD88 benchmark every square 177mi

5
Density Comparison AK WI
6
Effects of NAVD88 Network

• Vertical surveys very expensive, especially if
  performed off the road system
• Sparseness of adequate vertical control in remote
  AK leads to weak strength of figure in
  adjustments
• Vertical surveys impractical, especially if
  performed under NGS-58 and NGS-59 requirements

7
NGS-58 59

• Minimum 3 HARN (A or B order) within 75km of same
  or higher order
• Minimum of 3 Primary Bases within 50km of same or
  higher order
• Secondary Bases within 20km of same or higher
  order
• User Densification Network within 20km of same or
  higher order

For 5cm guidelines, 2cm is denser
8
OPUSDB

• Online publishing of GPS observations
• For the people by the people
• Linked to datasheets
• Allows for easy reconnaissance

9
OPUSDB Submissionsas of 2/21/2010
10
AdakOPUSDB DataSheet
Dave Doyle Not surprised that this Position is
more than 2m off published HZ He is surprised
that The orthometric height Is 4.328m from
NWLON LMSL
11
Time

• The position of a station is only truly known the
  moment of its last observation
• Stations are destroyed
• Stations move due to plate tectonics and
  isostatic rebound
• Stations move due to local geology

12
NAVD88 in Seward

• X74
• 72ft copper clad rod
• VERT ORDER - FIRST CLASS II
• Stability Code B

13
NAVD88 in Seward

• 945 5090 TIDAL 10
• Set in a massive structure
• VERT ORDER - FIRST CLASS II
• Stability Code B

14
NAVD88 in Seward

• Stability B Probably hold position/elevation
  well
• 2005 Required to level through these marks
  during annual maintenance of NWLON
• The two stations were 2.25 miles apart
• 0.234m The difference in Difference of
  Elevation between 2005 and 1967 published levels
• Which station moved?
• Perhaps both

15
NAVD88 in Seward
16
Uplift of the Kenai Peninsula

• Cumulative crustal uplift between 1964 and 1995
  relative to a site in Seward
• Cohen and Freymueller, 1997
• http//denali.gsfc.nasa.gov

17
Sea Level in Seward and Anchorage
18
Sea Level Falling in Nikiski
19
Land Rising at Nikiski
Copper clad Steel rod driven 12.2m (40ft)
20
Land Rising at Nikiski

• 24 hour annual sessions during NWLON maintenance
  trips
• NGS OPUS Solutions
• REF FRAME ITRF00
• 2006 Ellipsoid Height 15.975(m)
• 2007 Ellipsoid Height 16.006(m)
• 2008 Ellipsoid Height 16.055(m)
• 2009 Ellipsoid Height 16.044(m)

21
Sea Level is Falling in Skagway
22
Land Rising at Skagway
Copper clad Steel rod driven 5.8m (19ft)
23
Land Rising at Skagway

• 24 hour annual sessions during NWLON maintenance
  trips
• NGS OPUS Solutions
• REF FRAME ITRF00
• 2007 Ellipsoid Height 12.674(m)
• 2008 Ellipsoid Height 12.706(m)
• 2009 Ellipsoid Height 12.735(m)

24
New NWLON at Elfin Cove
25
Tidal BM Elevations at Elfin Cove

• Datasheets published less than 2 years apart
• Benchmarks rose 0.234m in 13.5years

26
Hooper Bay Tide Station
June 2007
September 2007

• SS Rod driven 8.53 m (28.0 ft) to refusal, June
  2007
• Ice lens collapsed the ground around the
  benchmark
• The benchmark did not move only the ground as
  confirmed through 3 wire levels

27
Conclusion

• Distribution and density of NAVD88 stations very
  sparse off the road system
• Benchmark elevations are dynamic with time user
  beware