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Ba thymetric Survey of the Mississipp i River Gulf Outlet (MRGO), GulfIntrac oa sta l Wa terwa y (GIWW), a nd Inner Ha rbor Naviga tion C ana l(IHNC) from Lake Pontc ha rtra in to Ba you La Loutre: Method ology,

Qua lity Co ntrol, and Error Ana lysis

Environm enta l Sc ienc e Servic es, Inc .P.O. Box 1289

Denham Springs, Louisiana 70727-1289

7 Ma rc h 2008

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INTRODUCTION

In order to p rovide up-to-da te d ata a s input for numeric al hydrodynam icmo d eling effo rts, a b a thym et ric survey o f the Mississipp i River Gulf Outlet (MRGO ), GulfIntrac oa sta l Wa terwa y (GIWW), and Inner Ha rb or Naviga tion Ca na l (IHNC) from La kePon tc ha rtra in to Ba you La Loutre wa s c ond uc te d 23 -27 Ja nua ry 2008 (Figure 1).

Figure 1. Sate llite ima ge show ing the survey transec ts and loca tions of Nationa l Geo de tic Survey(NGS) be nc hma rks oc c up ied d uring the survey. Benc hma rks a re p a rt o f the NGS LouisianaHeight Mod erniza tion Netw ork.

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METHODOLOGY

Planned LinesThe survey wa s p lanne d using C oa sta l Oc ea nog ra p hics HYPACK survey softwa re.

Cross-ch a nne l line sp a c ing wa s set a t 1000 ft (Figure 1). The p lanne d line files we re usedto na vig a te transec ts d uring t he b a thyme tric survey.Basic s of Data Ac quisition

The survey wa s c ond uc ted using a sing le-b ea m e c hosound er for de p thme a surements a nd p ost-p roc essed kinema tic g lob a l p osition ing system (GPS) forhorizonta l po sitions a nd eleva tions. During the p roc essing the d ep th m ea surem entsfrom the ec ho sou nd er are integ ra ted with the 3-d imensiona l XYZ GPS p osition t oc onve rt the d ep ths to referenc e a n Ea rth-ce ntered ellipsoid m od el, eliminat ing e rrorsa ssoc ia ted with wa ter surfac e eleva tion o r tid a lly-ba sed hyd rog ra p hic surveys. The fina lZ va lues a re in eleva tions rela tive to t he No rth Ame ric a n Vertica l Da tum of 1988 rev ision2004.65 [NAVD88(2004.65)]. Horizon ta l po sition s a re rep orted relative to North Am eric a nDa tum of 1983 revision 2007 [NAD83 (2007)].

National Geodetic Survey (NGS) Benchmarks and Base Station Hardware

GPS b a se sta tions we re set a t tw o NGS b enc hm a rks, Reg g io 2 a nd Waste Well 2,d uring the survey to p rovide a sta tic d a ta set w ithin the survey c ove ra g e a rea (Figures 2-5). Rover (survey ve ssel) GPS d a ta is referenc ed to the b a se sta tion d a ta d uring thep ost-proc essing. The b a se sta tions used a re p a rt o f the NGS Louisiana Heig htMo d erniza tion Projec t a nd refe renc e t he mo st rec en t revision (2004.65) of NAVD88. Thisis impo rta nt in South Louisia na b ec a use of the ra p id sub sidenc e ra tes in the reg ion. NGSb enc hma rk d a ta shee ts for Reg gio 2 and Waste Wa ter Well 2 are includ ed a s Ap p end ixA.

During survey d a ys the sta tic b a se sta tions a re set a b ove the NGS b enc hma rkson survey trip od s within 15 km of the a rea b eing surveyed . On d a ys wh en surveysextende d be yond the 15-km limit, a sec ond static ba se w as set o n a nother benc hma rk

to p rovide sea mless b a se sta tion co verag e for survey a rea s c om p leted within a d a y.Thus the e ntire survey a rea wa s c om p lete d b y seq uentia lly linking tog ethe r c ove ra g estha t we re o b ta ined within be nc hma rked G PS b a se sta tions.

Sta tic G PS d a ta including sa tellite p ositioning a nd sa tellite he a lth, wa sc ontinuously rec orde d a t 1-sec ond intervals using a n Ashtec h Z-Max.net d ua lfreq uenc y GPS rec eiver with a da ta logg er and a c hoke-ring a ntenna (Figures 2-3;Ap pe ndix C ). The rec eiver and ante nna c om bination rec ord 12-cha nnel full-ca rrier(L1/ L2) pha se p ositioning signa ls from the NAVSTAR GPS sa te llite c on ste lla tion. Antennaeleva tion wa s me a sured for inp ut d uring p ost-proc essing .

BA

Figure 2. A) Photog rap h ofsta tic GPS b a se set on NGSbe nc hma rk Reg gio 2. Notecho ke-ring a ntenna a ndsurvey trip od . B) Close-up

pho to o f tripod ba se set insta inless ste el rod a t sta tion

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Sound ing and GPS da ta we re strea me d in real-time to a n onb oa rd rugg ed izednoteb oo k com p uter for da ta log ging a nd naviga tion using HYPACK hydrog rap hicsurvey softwa re.

Post-Processing of Bathymetric DataPost-processing consists of two main steps: 1) kinematic GPS processing for rover

p ositioning a nd 2) integ ra tion of d ep th sound ings with po st-proc essed kinem a tic roverposition.

The first ste p is c a rried out using Wa yp oint Inc . GrafNa v softw a re. This softw a re isa static/ kinema tic ba seline p roc essing engine d esigne d to a c hieve ac c ura c ies d ow nto the c entime ter level and c an inc orporate multiple b a se-station d ata . GPS d atalog ge d a t the b a se sta tion, b a se sta tion ant enna height , a nd the NGS sp ec ified XYZc oo rdinate s for the b enc hma rk are inco rpo rate d with rover da ta to p roc ess kinema tictra jec tories (rove r po sitions). Gra fnav outp uts a single p rec ise b oa t p osition a nd q ua lityc ont rol informa tion a t 1-sec ond intervals.

The sec ond step is the me rg ing o f rove r position from Grafna v o utp ut w ith the

sound ings. In this step the sound ings a s log ged in HYPACK a re inp ut a long with b oa tpo sition, horizonta l and vertic al offsets be twee n a ntenna and tra nsd uce r fa c e, and anyc orrec tions ap plied b ased on e c hosound er ca libration d uring the survey (ba r chec k tob e d isc ussed in follow ing sec tion). The c orrec ted d ep th is c a lculate d b y:

D = (v * t) + d S + d GPS

Where v eq ua ls the a verag e ve loc ity of sound in wa ter co lumn , t eq ua ls the m ea suredelap sed time from tra nsd uc er to b otto m a nd b a c k to tra nsd uc er, d S is the o ffset fromtra nsd uc er fa c e to GPS a ntenna c ente r, and d GPS eq ua ls GPS ellipsoid height relativeto the GPS a ntenna c enter.

The ellipsoida l heights a re then c onve rted to orthom etric heights (NAVD882004.65) using the NGS GEOID03 revised 10/2005 ve rsion fo r southern Louisian a .

Data Quality ControlIn sha llow wa ter surveys (

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me a surement of the rela tive signa l streng th of the GPS sa te llite c onfigurat ion, mea suredinterna lly by the GPS system a nd d isp la yed a s a va lue typ ica lly be twe en 1 a nd 4. ThePDOP is a p roxy for po sition e rror, the low er the va lue the higher the a c c ura c y. WhenPDOP rea d ings excee de d a value of 3, ope ra tions we re ha lted , or da ta wa s rem ovedfrom the d a ta set d uring p ost-proc essing .

During p ost-p roc essing, GrafNa v d etermines a roo t me a n sq ua re (RMS) erro r forea c h p oint b a sed on GPS c yc le slip s, PDOP, sa te llite he a lth, and b a se sta tion RMS. AnyRMS va lues g rea ter tha n 0.14 m we re rem ove d d uring q ua lity c ont rol a nd a ssessme nt.Afte r rem oving o utliers, the fina l mea n RMS va lue wa s 0.07 m.

Figure 4. Histo gra m show ing d istrib ution or Roo t Mea n Sq ua re (RMS) va lues for the tot a lp op ulation o f survey p oints. The RMS is c a lculat ed in GrafNav softw a re ba sed on rove ra nd b a se sta tion sa tellite he a lth, Perc ent Dilution of Prec ision (PDOP), and b a se sta tionRMS. The me a n RMS for the survey w a s 0.07 m, a ny p o ints w ith a n RMS va lue g rea te rthan 0.14 m were eliminate d from the d a taset.

Wild Point Editing During wild -po int ed iting, the sound ing da ta a re p lott ed with the proc essed

survey e leva tion in NAVD88 versus time (Figure 5). Time is used a s a p roxy for d ista nc etraveled, as the survey speed was slow and consistent, rarely exceeding 4 knots, and

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the time, unlike the d istanc e traveled, a lwa ys mo ves forwa rd . Once p lotte d, the da tawe re ma nually evaluated and ed ited throug h the selection of po ints that w ere notrep resenta tive of the loc a l trend in the d a ta set. On a sep a ra te p lot, the Z va lue (GPS-derived elevation) for each point is plotted against the sounding (echosounder-derivedd ep th). Ide ally the p lot w ould ha ve a linea r trend , in c ases whe re po ints do not p lot o nthe line it is a ssumed tha t the y resulted from GPS error and a re rem ov ed . Erra nt p oints inthe d a ta set w ere likely the result of p oo r GPS signa l q ua lity, sea c ond ition s, ec hosound er returns from b elow t he sea b ed or from ob jec ts within the w a ter co lumn, suc ha s ma rine life.

Figure 5. Sc ree n c ap ture o f the w ild-po int editor program . The tw o p lots on the left show thesoun d ings in profile a s eleva tion ve rsus time. The u p p er one is ed itab le throug h g ra p hica l userinterfac e. The up p er right p lot show s the m a p v iew o f the survey tra nsec t. The low er right p lotshow s Z1 (GPS eleva tion) ve rsus Z2 (de pt h from ec hosound er on ly). Points tha t d o not p lota long the Z1 vs Z2 linea r trend c ont a in GPS errors a nd a re remove d from the d a ta set.

Data Va lida tion Throug h Crossline Chec k During the survey, an a long -cha nnel line trend ing p erpe nd icular to the c ross-

c ha nne l transec ts wa s surveyed . The a long -ch a nne l line c rossed ev ery transec tp roviding a n o p p ortunity to a ssess c onsistenc y of e levation m ea surem ent througho utthe stud y. At ea c h loc a tion whe re the rove r c rossed itself the differenc e in eleva tion(DZ) is calculated (Figure 6). Ideally, the processed elevation at any given crossing pointsho uld b e ide ntic a l (DZ=0). How ever, this is ra rely the c a se. An ide a l c rossing wouldhave two bo at p aths a c quiring a me a sureme nt at the exac t sam e p osition (Northinga nd Ea sting). How ever, this wa s a lso rarely, if ever, the c a se , so c omp a rative elev a tionme a surem ents we re c ond uc ted b y p roximity. A sc ript is the n run to : 1) id entify allc rossings, 2) interp olate the eleva tion for ea c h survey line a t the XY loc a tion o f tha t

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Figure 7. Histog ram show ing d istrib ution of DZ a t c rossings a fter ed iting . The me a ndifferenc e in e leva tion b etw ee n a ll c rossings is 0.0 m w ith a sta nda rd d eviation o f +/-0.14 m. This sa mp ling is a ssume d to b e rep resent a tive o f the ent ire p op ulation a ndtherefore p rovide s an estimate of the error assoc ia ted with the elevat ions rep orted

here.