Geoid Computation Difficulties in the Pacific Northwest

by

Dru A. Smith National Geodetic Survey

Presented at the 1st Official Meeting of the United States Geoid Committee Silver Spring, Maryland

August 18, 1998

Issues arising in Pacific Northwest geoid computations:

CV!-Terrain Corrections at NGS disagree with those at GSD(GC)

( -Terrain may not properly be represented by 30" data ~f~l/ fu -DEMs are not properly referenced to a consistent vertical

datum

qt6.. G96SSS agrees to decimeters with GPS/Benchmarks in PNW, but EGM96 {Bouguer corrected) disagrees to 1 meter with GPS/Benchmarks

Problem #1

TC differences, NGS vs GSD(GC)

1996 Study of Southern British Columbia

·.:

CANADIAN TERRAIN CORRECTIONS (Jan 93) Attemp~ !Q ~produce

- 2 DTEDs: - TOP030 - New Canadian DTED 1995

- 5 Independent TC programs: - ftc.f (FFT, Milbert) - tc01.f (Flat top Prism, Milbert) - tc.f (Flat top Prism, Forsberg) - tcpts01.f (Flat top Prism, Veronneau) - triter4.f (Inclined top Prism, Rupert/Beach)***

- 202 points in soo- 51 o N, 235.5°- 237.5°

*~* = Not fully tested yet . . - .. - . . . ....

~ .. ' .

Ave RMS Min Max

TC DIFFERENCES Jan93(Can Database) MINUS Other TCs

TOP030 1995 Canadian DTED 12 mgals 14 mgals 17 mgals 19 mgals -17 mgals -12 mgals +45 mgals +54 mgals

- 122 non-zero points - Overall stats for all 4 fully-tested programs - Conclusion: The Jan93 TCs {currently still in the Canadian database) are systematically higher than all 8 (4 programs, 2 DTEDs)TC sets at NGS,by a factor of 1.5 to 1.8

TC(Jan93) Minus TC(TOP030/Prlsm/200km)

-124.5 -123.5 -122.5

51.0 51.0 12.4

3.9 5. 9.3

-4.2 10.6 16.3 1.4-0.3 5.0 - 14.0 7

0.9 -2.1 14.6 4.7

-4.4 19.4 13.1

8 .6 13.4 6.5

39.8 9 .9 3 - 8 .8 16.5 23.3 7.3

0.0 0.3

(1) 15.1 11.7 6.4

"'0 8.5 ::J 17.2 +" 50.5 - -~.a> 2.8 50.5 +"

. lL8

ctS fi.Q -5.1 ..J 1&· 9 19.3 28.0 3.6 -4.0 18.5

30.8 22.2 3 .0 1.4 9.0

3.9 1.0

32.2 18.3 11. 3 8.7

·1~~7 4.5 4.0 7.7 -4.3 . 12.5

8 5 1.3 3.0

1.5 -8.3 -2.3

-5.7 -0.7 -2. - 16.6 17.7 17.4

0.1 18 .2 14

15.5 -4.6

4 .2 13.4

50.0 50.0

-124.5 -123.5 -122.5

Longitude

CANADIAN TERRAIN CORRECTIONS (Jan m Preliminary Results

- Unable to reproduce the January 1993 TC's

- Attempts using the old (TOP030) DTED gave results closer to Jan 1993 than the new DTED

- FFT method agrees to within +1- 1 mgal with prism methods, except for large (>30 mgal) spikes, where the FFT is systematically too low by an average of 8 mgals

~fr - Level 1 DTED (3 11X6 11

) unable to get Jan93 TCs! -M'-'1T·~:dr":. .. ...... . . ... ··-·-:--:-:,.,..-····---·-- ,, .:.:~~~~-~·~-... c---. ~ . i:~-.-·· M . , ···- ·-·· ·· ···- ···-MM ----:--~--.. - -~· .... - .... ·- -----,-~_ ... ;r-'::7- ·.~·:_:· --:-~-;-:-:--:-:-~·;-., ·. ' ·-··--~···· -- ··--------.. ····· -..... _. __ ....... .... . . ;.' ' (' ..

· ~·i· ... :. 'tY', . . .. .. ~ .· . _.,;_•.,:,._ . . •. . . ·"" . • • • 0 •

Problem #2

30 .. OEM fails !Q capture full terrain §!gnal

1996 Study of Southern British Columbia

Create Canadian TC's from 30 11 and 3 11 data

Examp~ point:

TC(Canada DB, W TC(Topo30) TC(3 11 X6 11)

(50.66067, 236.88400)

54.6 mgals 14.4 26.0

[This study will be re-investigated in August/September 1998]

Problem #3

No vertical datum consistency in available DEMs

1997/98 DEM/DTED Study

Sources Qf ~(or better) DEMs:

A) Most go back to 1960snos DMA 3 .. data - Old DMA data comes from 1 :250,000 maps

-1 :250,000 maps from old satellite sources - NIMA was updating cell by cell with new photo sources, but has nearly stopped recently

***NO vertical datum documentation or consistency

B) SRTM (expect new DTED in 2003) ***Vertical datum could be consistent!

C) USGS is digitizing 1 :24,000 maps onto 1 0 and 30 meter UTM grids (90°/o of west US done, 30o/o of East US done) ***Vertical datum well defined as NGVD29

/ , • ;:::. \ 1.1 ·.J II (? 0WJ ~q fr D ~ L-P () p \; , (:::.{J1/ i/v1 '\ -ti. vv

~- I -t Vc1{~'-0~ + Sb0 fFt1.JPr-f J 0(1\C{

· OEM Differences DMA 1998 DTED minus USGS 3 ..

(44° to 49° N, 237° to 243° E) .

RED = +25 m or greater differences MAGENTA= -25m or lower differences

Geoid Undulation changes due to random +200 meter error in one 1 °X1 o OEM (of 2'x2' elevations)

Red = + 1 em or greater change Magenta = -1 em or lower change

Geoid Undulation changes due to systematic +20 meter error in one 1 °X1 o OEM (of 2'x2' elevations)

Red = + 1 em or greater change Magenta = -1 em or lower change

Problem #4

EGM96 bust in Pacific Northwest

1996/97 GEOID96 computation and validation

G96SSS minus EGM96(Bouguer Corrected)

Red=+ 1.5 meters and greater differences Magenta=-1.5 meters and lesser differences

EGM96 and G96SSS vs. GPS on Benchmarks

-Compared both models to ITRF94/NAVD88 data

- National average residuals of: EGM96/GPS/BM: +41 em G96SSS/GPS/BM: +43 em

In PNW (44°-49°, 237°-243°), with average removed: EGM96/GPS/BM: +94 em + 28 em G96SSS/GPS/BM: -12 em ± 19 em

Q)

"C ::J ..... ..... co ..J

GPS/BM/G96SSS residuals (about 43 em ave)

-123.0 -120.0 -117.0 49.0,_~~~===========-========~==========~~~~============~~~=====r

-3c.8 • 49.0

•

-25 .5 •

-\1.4 -18.2 -15._58 ~

-~.2 . .

14.4

21.4 •

26.8

: ~~·ia86 ••• 30.1

•

29.6

- 1.6 • - n._ 3

J!.:a!

•

•

16.6

33.6

-42.9 •

• -3 0.3 • -38 .4

• -27.0 -32.1 •

• :41.8 .. ~~~~i

-3'7~0.9 • ·-27.5

-29.4 • • _32.5 .-.s.OfeS~~~j2~.a21.2

• 2A 2 - .-. z4.9 - 11 .5

:2J1~11:tf!!l-1~~.6 :21.6 : 14.7• • • ·;t:f~~6.4 • :·4 • .. ~

~ • -'tt;.9 • 5.7 -7·3 .-3.9 1~

.: 1.~ :-~?:J: 1 ~. 9 ..5 1 • -If. .0

1.9 4o.r· - 3 1 • • Q:§ .O . . . . • 6.7

• - 3.3

•

5.0 •

19.9 • 25 .0

•

• •

-50.'1 •

-46.4 •

-53. •

50 .0 •

-41.6 ._44. 7 -27.2 •

-38.1

- 23.7 -25.4 - 3 .0 • •

-20.6 •

-17.7 •

- 15.5 •

-17.6.-20.5 •

-14 .6 •

-4.9 •

-22.1 •

-27.1 •

32.4 • 38.0 •

44.0~--------------------~----------------~---------------------r--------------------~--------------------~-----------------+ 44.0 -117.0 -123.0 -120.0

Longitude

Q)

"C :::J ...., ·-...., as ~

GPS/BM/EGM96 residuals {about 41 em ave)

-123.0 - 120.0 -117.0 49.01-~~~q===~==========~==========~==--1~0=1.~9~d6==========d===.~~====~ 49.0

109 .9

148.3 •

97.7

: e~bm •••

90.2 •

16.6 •

w.o 52.9 .101.4~~-7

50.5 • .s!.\8

157.9 •

82.1 •

•

• 118.8 48.9 • 70.1 • • 104.2

63.9 • • • 63.4 •• 1~i~~

• 93.~8.6 • H~:o •

115.7

28.9 • 80.9 •

97.1 •• 8.~,8~~55p 90.2

• • 1 H~3 ~C\1A 4$~2 • 82 ,1 109t1 101•8

•• ~89~fai9~· 4.5 • • • • 411

••• ;c .. .m.6 ~ • 0 7'

• 115.1 1,2.5 .• 112.0 7.

: 153.0 •• ;~ .. 4~~% 4:1.5 1.15. .

• 166.-61.72 .~ 113.2 •• 1.1 ~8.1

•

•

• 94.7

•

• 80.5

•

66 .4 •

91.6 •

104.8 •

• 91.6

61.1

64. •

• 87.6

• 92.2 •

92. •

60.4

111.5 106.3 11b.4 .3 • • 105.4

• 91 .7 •

108.5120.3 •

109.8 112.2 • 130.2 •

• 111.0 •

121.6

107.1 •

44.0~--------------------~--------------------~--------------------~-----------------,--------------------~--------------------~ 44.0 -117.0 -123.0 - 120.0

Longitude

CONCLUSIONS

- Most geoid research effort is concentrating on the Pacific Northwest

- Many problems stem directly from unreliable high resolution DEMs

- GPS on Benchmarks provide a useful independent check on geoid models

- Additional research being done on downward continuation and long wavelength terrain effects