Prepared in cooperation with the

City of Ithaca, New York, and the New York State Department of State

Bathymetric Survey of the Cayuga Inlet

Flood-Control Channel and Selected

Tributaries in Ithaca, New York, 2016

Open-File Report 2017–1109

U.S. Department of the Interior

U.S. Geological Survey
Cover. “Upstream view of Cayuga Inlet, Ithaca, New York”

(Photograph by William F. Coon, U.S. Geological Survey)
Bathymetric Survey of the Cayuga Inlet 

Flood-Control Channel and Selected 

Tributaries in Ithaca, New York, 2016

By John F. Wernly, Elizabeth A. Nystrom, and William F. Coon

Prepared in cooperation with the

City of Ithaca, New York, and the New York State Department of State

Open-File Report 2017–1109

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

William H. Werkheiser, Acting Director

U.S. Geological Survey, Reston, Virginia: 2017

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit https://store.usgs.gov.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Wernly, J.F., Nystrom, E.A., and Coon, W.F., 2017, Bathymetric survey of the Cayuga Inlet flood-control channel and selected tributaries in Ithaca, New York, 2016: U.S. Geological Survey Open-File Report 2017–1109, 9 p., 

https://doi.org/10.3133/ofr20171109.

ISSN 2331-1258 (online)
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Bathymetric Survey.......................................................................................................................................3

Survey Plan............................................................................................................................................4

Horizontal and Vertical Control...........................................................................................................4

Postprocessing of Bathymetric Data ................................................................................................6

Horizontal and Vertical Control Accuracy ........................................................................................6

Creation of Bathymetric Surface ................................................................................................................7

Summary..........................................................................................................................................................7

Acknowledgments.........................................................................................................................................7

References Cited............................................................................................................................................9

Figures

1. Map showing locations of channels included in the bathymetric survey of the

Cayuga Inlet flood-control channel and selected tributaries, Ithaca, New York. .............2

2. Map showing locations of transects for echo-sounder depth measurements from

the bathymetric survey of the Cayuga Inlet flood-control channel and selected

tributaries, Ithaca, New York ......................................................................................................5

3. Map showing bathymetric surface of the Cayuga Inlet flood-control channel and

selected tributaries, Ithaca, New York......................................................................................8
iv

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2)

Volume

quart (qt) 0.9464 liter (L) 

Velocity

foot per second (ft/s) 0.3048 meter per second (m/s)

Mass

ounce, avoirdupois (oz) 28.35 gram (g) 

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Datum

Vertical coordinate information is referenced to either the National Geodetic Vertical Datum of 

1929 (NGVD 29) or the North American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.

Supplemental Information

Concentrations of chemical constituents in water are given in parts per thousand, which is 

equivalent to grams per liter.
v

Abbreviations

ADCP acoustic Doppler current profiler

GIS geographic information system

GPS Global Positioning System

RMSE root mean square error

RTK real-time kinematic

TIN triangular irregular network

USACE U.S. Army Corps of Engineers

USGS U.S. Geological Survey

Bathymetric Survey of the Cayuga Inlet

Flood-Control Channel and Selected

Tributaries in Ithaca, New York, 2016

By John F. Wernly, Elizabeth A. Nystrom, and William F. Coon

Abstract

From July 14 to July 20, 2016, the U.S. Geological Survey, in cooperation with the City of Ithaca, New York, and the 

New York State Department of State, surveyed the bathymetry 

of the Cayuga Inlet flood-control channel and the mouths 

of selected tributaries to Cayuga Inlet and Cayuga Lake in 

Ithaca, N.Y. The flood-control channel, built by the U.S. Army 

Corps of Engineers between 1965 and 1970, was designed to 

convey flood flows from the Cayuga Inlet watershed through 

the City of Ithaca and minimize possible flood damages. Since 

that time, the channel has infrequently been maintained by 

dredging, and sediment accumulation and resultant shoaling 

have greatly decreased the conveyance of the channel and its 

navigational capability.

U.S. Geological Survey personnel collected bathymetric 

data by using an acoustic Doppler current profiler. The survey 

produced a dense dataset of water depths that were converted 

to bottom elevations. These elevations were then used to 

generate a geographic information system bathymetric surface. 

The bathymetric data and resultant bathymetric surface show 

the current condition of the channel and provide the information that governmental agencies charged with maintaining the 

Cayuga Inlet for flood-control and navigational purposes need 

to make informed decisions regarding future maintenance 

measures.

Introduction

The Cayuga Inlet basin covers 158 square miles (mi2) 

that drain to the southern end of Cayuga Lake, where the 

City of Ithaca, Tompkins County, New York, is located. The 

inlet, which flows northward, has two tributaries, Sixmile and 

Cascadilla Creeks, that flow westward through Ithaca to join 

Cayuga Inlet 1.4 and 0.6 miles (mi), respectively, upstream 

from its mouth. A third stream, Fall Creek, also flows through 

Ithaca but discharges directly into Cayuga Lake (fig. 1). 

Although infrequent, these four streams, either individually or 

collectively, have the potential to cause great damage to city 

properties and infrastructure when they flood. The magnitude 

and extent of flooding can be affected by the water level of 

Cayuga Lake; high lake levels prevent drainage of flooded 

areas through storm-sewer pipes, which under extremely high 

water levels can be the source of flooding as water backs up 

through these conduits. Flood extents can also be affected 

by any decrease in the conveyance of storm flows through 

Cayuga Inlet.

The 2.3-mi-long Cayuga Inlet flood-control channel 

was built by the U.S. Army Corps of Engineers (USACE) 

between 1965 and 1970 to convey flood flows through the 

City of Ithaca and minimize flood damages similar to those 

that occurred during the inlet’s period-of-record flood on July 

8, 1935 (U.S. Army Corps of Engineers, 2011; New York 

State Department of Environmental Conservation, 2016). 

However, with time, this design goal has been thwarted by (1) 

sedimentation and shoaling in the low-gradient, over-widened, 

and over-deepened channel that is also under the backwater 

influence of Cayuga Lake, and (2) the lack of maintenance 

dredging during the channel’s 46-year existence (U.S. Army 

Corps of Engineers, 2011). Dredging of the inlet has become 

a critical need for the City of Ithaca, and a realistic estimate 

of the volume of sediment that might need to be removed is 

required. The first step toward that estimate is an accurate map 

of the channel bottom that could then be compared with asbuilt flood-control channel (circa 1970) cross-sectional data. 

Mapping also is integral to flood-inundation modeling of the 

Cayuga Inlet channels. 

The U.S. Geological Survey (USGS) provides reliable 

scientific information to describe hydrologic systems and 

assists in understanding how hydrologic and geomorphic 
2 Bathymetric Survey of the Cayuga Inlet Flood-Control Channel and Selected Tributaries in Ithaca, New York, 2016

Cayuga Lake Cayuga Inlet

Fall Creek

ITHACA

Sixmile Creek

Old Cayuga Inlet

Cayuga Inlet

Creek

Cascadilla

Base map image is the intellectual property of Esri and is used herein under license.

Copyright © 2014 Esri and its licensors. All rights reserved.

EXPLANATION

U.S. Geological Survey

streamgage and number

04233500

NEW YORK STATE ROUTE 13

NEW

YORK

STATE ROUTE 89

04233500

0.25 0.5 MILE

0 0.25 0.5 KILOMETER

0

76°31'30" 76°31' 76°30'30" 76°30'

42°27'30"

42°27'

42°26'30"

42°26'

Study area 

NEW YORK

Location map

CAYUGA LAKE

SENECA LAKE

Figure 1. Locations of channels included in the bathymetric survey of the Cayuga Inlet flood-control channel and selected 

tributaries, Ithaca, New York. The blue lines on the base image are enhancements to highlight the stream channels.
Bathymetric Survey 3

conditions can change with time. In most instances, the USGS 

documents and analyzes the effects of widely recognized 

phenomena, such as surface-water and groundwater flow and 

quality. On occasion, this information involves small-scope 

studies of recognized phenomenon, such as shoaling of the 

Cayuga Inlet flood-control channel, which had not been previously documented at a scale to adequately meet the needs of 

governmental agencies, including the USACE, the New York 

State Department of Environmental Conservation, and the 

City of Ithaca. The USGS works within its strategic science 

direction and with its cooperative partners, such as the City of 

Ithaca, to document emerging environmental conditions and 

(or) hazards and to ensure that scientific methods are applied 

effectively to better understand these phenomena and thereby 

inform environmental managers on water resources and associated infrastructure.

The USGS, in cooperation with the City of Ithaca, N.Y., 

and the New York State Department of State, as a project of 

the Title 11 of the Environmental Protection Fund, surveyed 

the bathymetry of the Cayuga Inlet flood-control channel, 

including the Old Cayuga Inlet channel; the downstream ends 

of two tributaries to Cayuga Inlet, Sixmile and Cascadilla 

Creeks; and the downstream end of one tributary to Cayuga 

Lake, Fall Creek. The survey extents of each of these tributary reaches was from their respective mouths upstream to 

New York State Route 13 in Ithaca (fig. 1). The survey was 

conducted to meet the modeling needs of a concurrent floodinundation mapping study that was being conducted by the 

USGS. The bathymetric data also provided a picture of the 

current (2016) condition of the flood-control channel, which 

can inform government agencies charged with maintaining the 

depth of the flood-control channel to design specifications and 

the navigational capacity of the inlet as part of the New York 

State Canal System.

The purpose of this report is to document the procedures followed during collection of bathymetric data from the 

flood-control channel of Cayuga Inlet, as well as the mouths of 

Sixmile Creek, Cascadilla Creek, and Fall Creek. The report 

describes the collection of depth data by an acoustic Doppler 

current profiler (ADCP), the processing of the depth data, 

and the assessment of the accuracy of the depth measurements. A map of the bathymetric surface of the inlet and the 

three tributaries is presented. These data are available as a 

separate data release through USGS ScienceBase (Wernly and 

Nystrom, 2016).

Bathymetric Survey

The main component of the bathymetric survey is the 

2.3-mi-long Cayuga Inlet flood-control channel from the railroad bridge and weir at the upstream end of the channel to its 

mouth at Cayuga Lake (fig. 1). Secondary components of the 

survey are the Old Cayuga Inlet channel from its intersection 

with Sixmile Creek to its confluence with the flood-control 

channel and the downstream ends of Sixmile, Cascadilla, 

and Fall Creeks from their respective mouths upstream to 

New York State Route 13.

Bathymetric data were collected with an ADCP, specifically a SonTek RiverSurveyor M9 with a 0.5-megahertz (MHz) integral vertical-beam echo sounder (SonTek, 

2015a). In the shallowest areas of the surveyed reach, data 

were collected using the RiverSurveyor M9’s dual four-beam 

velocity measurement transducers with four beams at 3.0 MHz 

and four beams at 1.0 MHz, and with both in Janus configuration and set at a 25-degree slant angle. An echo sounder measures water depths by measuring the time interval required for 

ultrasonic waves to travel, at a known velocity, from a known 

point (transducer face) to a reflecting surface (channel bottom) 

and then return. The dual four-beam transducers measure 

water depth by the same means, taking into account their slant 

angle. When either the 1.0-MHz or 3.0-MHz transducers are 

operating (only one frequency is operational at a time), four 

depths are measured (one per beam). The reported depth value 

is the average of the four measured depths.

 A minimum depth limitation applies when the reflecting 

surface is too close to the echo-sounder face. The minimum 

depth limitation using the vertical-beam echo sounder was 

generally less than about 1.9 feet (ft) during the survey. With 

the dual four-beam transducers, however, it was possible to 

decrease the minimum depth limitation to 1.4 ft. This decrease 

in the minimum measureable depth permitted the extension 

of the survey to shallow areas, particularly the most upstream 

portions of the tributaries where water depths decreased as the 

elevations of the streambeds increased. Survey points were 

georeferenced using a real-time kinematic (RTK) Global Positioning System (GPS) that is a component of the ADCP.

The velocity of sound in water is dependent on the salinity, temperature, and depth. The salinity and shallow depths in 

freshwater bodies are not major factors that affect the sound 

velocity (U.S. Army Corps of Engineers, 2002). Regardless, 

salinity was measured daily at the surface and at a depth of 

5 ft; additional measurements were made at 10 ft when the 

depth permitted. Salinities at the surface and at both depths 

were determined to be identical on a given survey day and 

ranged from 0.2 to 0.3 parts per thousand among the survey 

days. An ADCP has built-in temperature sensors that measure water temperature at the transducer face and permits the 

computation of the sound velocity and the subsequent accurate 

measurement of depth. To confirm the accuracy of the ADCP’s 

sound-velocity calculation, surface-water temperature was 

measured independently by means of a thermistor. Throughout 

the survey the ADCP temperature sensor varied by no more 

than 0.7 degrees Fahrenheit (°F) (0.4 degrees Celsius, °C) 

from the independently measured surface-water temperatures, 

with a mean absolute difference of 0.4 °F (0.2 °C). The water 

temperatures recorded at depths of 5 and 10 ft varied by no 

more than 3.1 °F (1.7 °C) from the measured surface temperature, with a mean absolute difference of 1.3 °F (0.7 °C). 

Therefore, no variation in sound velocity with depth, as a 
4 Bathymetric Survey of the Cayuga Inlet Flood-Control Channel and Selected Tributaries in Ithaca, New York, 2016

function of change in temperature, was expected in this shallow environment.

Before surveying each day, the depth readings of the 

vertical-beam echo sounder were checked by means of a bar 

check to ensure accurate depth sounding. A horizontal plate 

was suspended at known depths—5 and 10 ft—directly below 

the transducer face. The depth indicated by the vertical-beam 

echo sounder signal that was reflected off the submerged plate 

was compared with the known depth of the plate. The bar 

check was not used to ensure accurate depth soundings for the 

dual four-beam transducers.

All data were referenced to the horizontal datum, North 

American Datum of 1983 (NAD 83), and vertical datum, 

North American Vertical Datum of 1988 (NAVD 88). The 

survey was conducted from July 14 to July 20, 2016.

Survey Plan

Guidelines for conducting a bathymetric survey as 

described by Wilson and Richards (2006) were followed. The 

planned survey transects were arranged perpendicular to the 

centerline of each channel. HYPACK software (HYPACK, 

Inc., 2015) was used to map the transects, along which water 

depths were measured, over the length of the inlet and thus 

enabled the collection of consistently spaced water depths. 

Transects composed of series of survey points were spaced 

about 100 ft apart (fig. 2). A georeferenced aerial image of 

the channel was used within HYPACK for office planning 

of transects and for guidance of the survey team in the field. 

The survey team used an onboard differential GPS (separate 

from the RTK GPS that is a component of the ADCP) that was 

coupled to a computer, on which HYPACK was running, to 

navigate the transects in real time and keep track of their datacollection progress.

For quality assurance, a second survey was conducted 

to provide an independent dataset with which the vertical 

accuracies of the ADCP data could be assessed (fig. 2). This 

quality-assurance survey, which was conducted during the 

same time period as the principal survey, was run down the 

center of the channel. At every intersection between the first 

and second surveys, a coincident point was later extracted for 

depth-accuracy analysis.

Horizontal and Vertical Control

Horizontal survey positions for the ADCP were acquired 

through the use of an RTK GPS receiver positioned directly 

above the ADCP where depths were measured. This unit 

received horizontal-location corrections from an onshore RTK 

base station, which was periodically repositioned to maintain 

line of sight and thus maintain reliable communication with 

the survey vessel and provided a manufacturer-stated horizontal survey accuracy of 3 centimeters (cm) or 0.10 ft relative to 

the base station. The software associated with the RiverSurveyor M9 ADCP, RiverSurveyor LIVE (version 3.8; SonTek, 

2015b), does not allow input of a control-point location for the 

base station, nor does it allow for the logging of observations 

that could be postprocessed by the National Geodetic Survey 

Online Positioning User Service (National Geodetic Survey, 

2015). RiverSurveyor LIVE is designed for measuring streamflow where the absolute georeferenced location of the ADCP 

is not critical to a precision of plus or minus 3 cm. The system 

uses the base station to correct the ADCP position to plus or 

minus 3 cm relative to itself for the purpose of accurately measuring stream width to calculate discharge. This system was 

adapted for bathymetric mapping by obtaining a high level 

of accuracy for the ADCP relative to the position of the RTK 

base station, whereas, without the ability to enter a controlpoint location for the RTK base station, the absolute position 

of the ADCP may only have attained submeter accuracy.

Latency is the time difference or lag between the time 

when positioning data are received and the time when the 

computed or processed position reaches the data logging module and is time tagged. Latency typically results in a negative 

along-track spatial displacement of depth measurements. This 

displacement can be minimized by surveying at slow speeds 

and by synchronizing the echo-sounder and GPS clocks. Boat 

speed was kept to a minimum and averaged 0.78 feet per second throughout the survey. The RiverSurveyor M9 ADCP 

minimizes latency through the use of the integrated RTK GPS 

located above the ADCP, which directly connects the incoming GPS data stream to the ADCP. Unlike other echo-sounder 

systems, the RiverSurveyor M9 ADCP performs internal 

calculations with the GPS data, synchronized by the system 

with a 10-hertz update rate, before the data are stored in the 

computer. Latency was not quantified, but it was considered 

small enough, given the slow survey speed and integrated 

GPS, that any potential errors in the data were negligible for 

this survey.

A vertical control point for the survey was provided by 

the USGS streamgage, Cayuga Inlet (Cayuga Lake) at Ithaca 

(station number 04233500; U.S. Geological Survey, 2016). 

The streamgage, located approximately midway along the 

study reach (fig. 1), continuously records the water-surface 

elevations of the inlet. The daily surveys were conducted on 

relatively calm days; water levels during a given survey period 

fluctuated by 0.10 ft or less. The average of the water-surface 

elevations that were recorded during a given survey period 

was assumed to be constant throughout the survey reach. The 

mean water-surface elevations varied by only 0.15 ft during the 4 days that the survey was conducted. Water-surface 

elevations recorded by the streamgage are referenced to 

National Geodetic Vertical Datum of 1929 and were converted to elevations above North American Vertical Datum 

of 1988 by using the datum conversion program, VERTCON 

(National Geodetic Survey, 2016). During postprocessing 

of the data, water depths (adjusted for the transducer depth) 

were subtracted from these water-surface elevations to obtain 

channel-bottom elevations.
Bathymetric Survey 5

Cayuga Lake Cayuga Inlet

Fall Creek

ITHACA

Sixmile Creek

Old Cayuga Inlet

Cayuga Inlet

Creek

Cascadilla

0.25 0.5 MILE

0 0.25 0.5 KILOMETER

Base map image is the intellectual property of Esri and is used herein under license. 0

Copyright © 2014 Esri and its licensors. All rights reserved.

EXPLANATION

Survey points

Quality-assurance points.

(Individual dots in enlarged

image)

NEW YORK

STATE ROUTE 13

NEW

YORK

STATE ROUTE 89

76°31'30" 76°31' 76°30'30" 76°30'

42°27'30"

42°27'

42°26'30"

42°26'

Figure 2. Locations of transects for echo-sounder depth measurements from the bathymetric survey of the Cayuga Inlet 

flood-control channel and selected tributaries, Ithaca, New York.
6 Bathymetric Survey of the Cayuga Inlet Flood-Control Channel and Selected Tributaries in Ithaca, New York, 2016

Postprocessing of Bathymetric Data

Raw transect data were reviewed in RiverSurveyor 

LIVE, the software associated with the RiverSurveyor M9 

ADCP (SonTek, 2015b), for erroneous measurements that 

could result from false reflections from fish, aquatic vegetation, or other submerged objects. In addition, those data that 

did not meet RTK quality, as sometimes occurred when the 

GPS signal was lost near bridges or line of site with the base 

station was impeded, were also identified. Of a total of 26,976 

data points, 1,414 erroneous data points, or about 5.2 percent 

of the total survey, were removed. Most of these erroneous 

points were removed because they did not meet RTK quality, but a small number were removed due to interference 

from aquatic vegetation in the water column. The remaining 

transect data, consisting of 25,562 data points, were saved as 

edited transects.

The resultant data, after water depths were converted to 

channel-bottom elevations, were processed by using ArcGIS (version 10.3.1), which contains hydrographic software 

for data viewing and editing, data processing, generation 

of interpolated data, and creation of map products (Esri, 

2016). These data are available as a data release (Wernly and 

Nystrom, 2016).

Horizontal and Vertical Control Accuracy

The horizontal positional accuracy of the RTK GPS 

receiver in the ADCP is 3 cm (0.10 ft) according to the 

manufacturer (SonTek, 2015a); however, horizontal accuracy 

is affected by the multipath environment (proximity to trees, 

buildings, cliffs, and so on), the number of satellites in view, 

satellite geometry, and ionospheric activity (SonTek, 2015a). 

No independent testing was conducted to confirm the stated 

horizontal accuracy of the survey, but for the purposes of this 

study, the exact location of a given surveyed point in space 

was deemed to be less important than the relative locations 

among the surveyed points and the distances between adjacent 

points; therefore, the manufacturer’s level of accuracy was 

accepted as stated.

The vertical-beam echo sounder and the dual four-beam 

transducers used in this study have a resolution of 1 millimeter (0.003 ft) and an accuracy of 1 percent of the measured 

depth, according to the manufacturer’s specifications (SonTek, 2015a). A decrease in the accuracy of the average depth 

measured by the four-beam transducers can occur simply 

because the transducer beams are at a 25-degree slant angle. In 

all but perfectly flat environments, each of the four beams will 

measure and record slightly different depths with the average 

of the four depths being the only reported value. As the depth 

of water increases, the areal footprint being measured by the 

four beams increases, which can potentially lead to greater 

differences in measured depths among the four individual 

beams. For this reason the four-beam depths were generally 

used only where depths were shallower than the vertical-beam 

echo sounder’s minimum depth limitation of 1.90 ft. In these 

situations the areal footprint of the four-beam transducers was 

smallest and the potential error in depths was least. Four-beam 

transducer depths consisted of only 335 survey points or about 

1.3 percent of the total of 25,562 points measured.

Central to this project was the belief that sedimentation 

near the mouths of the tributaries was as important as the 

current level of shoaling of Cayuga Inlet. Four-beam depth 

measurements provided data in the shallowest parts of the survey areas that otherwise would not have been mapped. These 

data more accurately defined shoaling in the Old Cayuga Inlet 

channel and the mouth of Cascadilla Creek and also enabled 

the survey to continue an additional 300 ft upstream on Fall 

Creek, which would not have been possible with the verticalbeam echo sounder. Four-beam depth measurements also were 

made along the shallow edges of Cayuga Inlet.

An assessment of the vertical accuracy of the measured 

depths and resultant channel-bed elevations was possible 

by comparing results for coincident points measured in the 

primary set of transects (100-ft spacing though the entire study 

reach) and the quality-assurance survey (following a path 

along the center of the channel). One-hundred and fifty-five 

(155) paired intersection points were used for comparison. The 

horizontal distances between the paired points averaged 1.09 ft 

and ranged from 0.18 to 1.60 ft. The absolute differences in 

elevations ranged from 0.00 to 0.88 ft; however, all except 14 

paired points had differences within 0.20 ft and the median 

absolute difference was 0.07 ft. The vertical root mean square 

error (RMSE) and the vertical accuracy of the elevations at 

the 95-percent confidence level were computed by using the 

following equations:

∑

n

( ) Zorig 2 i i − Zqa

RMSE = i=1 z

n , (1)

where

 RMSEz is the vertical root mean square error,

 Zorigi is the vertical coordinate of the ith 

check point in the original dataset,

 Zqai is the vertical coordinate of the ith check 

point in the quality-assurance dataset,

 i is an integer from 1 to n, and

 n is the number of points being checked.

 Az

 = 1.960 × RMSEz, (2)

where

 Az is the vertical accuracy calculated at the 

95-percent confidence level.

The RMSE of the ADCP-collected elevation data was 0.14 ft, 

and the vertical accuracy was 0.27 ft at the 95-percent confidence level.
Creation of Bathymetric Surface 7

Creation of Bathymetric Surface

The Cayuga Inlet is about 150 ft wide in the upstream 

half of the study reach (that is, upstream from New York State 

Route 89). This width gradually increases through the downstream half of the reach to more than 400 ft near its mouth at 

Cayuga Lake. Midchannel depths typically ranged from 3.2 ft 

to 13.5 ft in the upstream part of the study reach, and 5.7 ft to 

10.3 ft in the downstream part of the study reach. The greatest 

depth, 14.5 ft, was measured near the right bank of Cayuga 

Inlet near its confluence with Sixmile Creek.

The bathymetric data were used to produce a triangular 

irregular network (TIN) of the bottom of the Cayuga Inlet 

flood-control channel, including the Old Cayuga Inlet channel and the mouths of Sixmile Creek, Cascadilla Creek, and 

Fall Creek (fig. 3). The TIN was created by connecting each 

point with its two nearest neighbors; for point data collected 

along transects, this creates a three-dimensional array of 

thin triangles. Breaklines that enforce connections between 

surveyed points were added to the computer-generated TIN 

to ensure accurate representation of the linear features of 

the channel bottoms (“linear enforcement,” see Wilson and 

Richards, 2006). The TIN, which was created using ArcGIS 

software (Esri, 2016), was projected in a State plane (New 

York Central) coordinate system. From this TIN, a 2-ft cellsize raster was created using linear interpolation. This raster 

and the bathymetric data are available online (Wernly and 

Nystrom, 2016). These data and the generated bathymetric 

surface are not intended for navigation, regulatory, permitting, 

or other legal purposes. The USGS assumes no legal liability 

or responsibility resulting from the use of this information.
8 Bathymetric Survey of the Cayuga Inlet Flood-Control Channel and Selected Tributaries in Ithaca, New York, 2016

76°31'30" 76°31' 76°30'30" 76°30'

42°27'30"

42°27'

42°26'30"

42°26'

Cayuga Lake Cayuga Inlet

Fall Creek

ITHACA

Sixmile Creek

Old Cayuga Inlet

Cayuga Inlet

Creek

Cascadilla

0.25 0.5 MILE

0 0.25 0.5 KILOMETER

0 Base map image is the intellectual property of Esri and is used herein under license.

Copyright © 2014 Esri and its licensors. All rights reserved.

This figure is not intended for navigation, regulatory, permitting, or other legal purposes. 

The USGS assumes no legal liability or responsibility resulting from the use of this information. 

EXPLANATION

Elevation, in feet 

(NAVD 88)

382.6 ± 0.3

367.6 ± 0.3

NEW YORK

STATE ROUTE 13

NEW

YORK

STATE ROUTE 89

Figure 3. Bathymetric surface of the Cayuga Inlet flood-control channel and selected tributaries, Ithaca, New York.
References Cited 9

Summary

A bathymetric survey of the Cayuga Inlet flood-control 

channel, including the Old Cayuga Inlet channel and the 

mouths of Sixmile Creek, Cascadilla Creek, and Fall Creek in 

Ithaca, New York, was conducted by the U.S. Geological Survey (USGS), in cooperation with the City of Ithaca, N.Y., and 

the New York State Department of State, during July 14–20, 

2016. The bathymetric data were collected with an acoustic 

Doppler current profiler along transects spaced about 100 feet 

apart along the study reach. Horizontal control of the surveyed 

points was maintained by the use of an onboard real-time 

kinematic Global Positioning System and an onshore RTK 

base station. A quality-assurance survey provided a second 

set of data, which enabled assessment of the accuracy of the 

measured depths. A USGS streamgage in the study reach provided water-surface elevations from which water depths were 

subtracted to obtain channel-bottom elevations. Geographic 

information system software was used to create a bathymetric 

surface from the bottom elevations. The bathymetric data and 

surface are available online as a separate data release.

The survey was conducted to meet the modeling needs 

of a concurrent flood-inundation study by the USGS. The 

bathymetric data can also be used to inform government agencies charged with maintaining the depth of the flood-control 

channel to design levels and the navigational capacity of the 

Cayuga Inlet as part of the New York State Canal System.

Acknowledgments

The authors would like to recognize Nolan Milliman of 

the U.S. Geological Survey for his skillful piloting of the survey boat, which permitted accessing and surveying difficultto-reach areas of the study reach.

References Cited

Esri, 2016, ArcGIS: Esri website, accessed September 17, 

2016, at http://www.esri.com/software/arcgis/.

HYPACK, Inc., 2015, HYPACK hydrographic survey software, version 15.0: HYPACK, Inc., website, accessed 

November 2015 at http://www.hypack.com.

National Geodetic Survey, 2015, OPUS—Online Positioning 

User Service: National Geodetic Survey database, accessed 

November 2015 at http://www.ngs.noaa.gov/OPUS/.

National Geodetic Survey, 2016, VERTCON—North American vertical datum conversion: National Geodetic Survey, 

accessed September 17, 2016, at http://www.ngs.noaa.gov/

TOOLS/Vertcon/vertcon.html.

New York State Department of Environmental Conservation, 

2016, Ithaca flood damage reduction project: New York 

State Department of Environmental Conservation, accessed 

on September 7, 2016, at http://www.dec.ny.gov/docs/

water_pdf/fcpprjithaca.pdf.

SonTek, 2015a, RiverSurveyor S5 and M9: SonTek web page, 

accessed June 20, 2016, at http://www.sontek.com/

productsdetail.php?RiverSurveyor-S5-M9-14.

SonTek, 2015b, RiverSurveyor LIVE (RSL): SonTek web 

page, accessed June 20, 2016, at http://www.sontek.com/

softwaredetail.php?RiverSurveyor-LIVE-RSL-34.

U.S. Army Corps of Engineers, 2002, Engineering and 

design—Hydrographic surveying manual: U.S. Army Corps 

of Engineers EM 1110–2–1003, 510 p.

U.S. Army Corps of Engineers, 2011, Hydraulic analysis and 

impacts of long term shoaling for Flood Risk Management 

Project, Cayuga Inlet, Ithaca, New York: U.S. Army Corps 

of Engineers, Buffalo District, 13 p. plus appendices.

U.S. Geological Survey, 2016, USGS 04233500 Cayuga 

Inlet (Cayuga Lake) at Ithaca, NY: U.S. Geological Survey 

National Water Information System database, accessed 

September 17, 2016, at https://waterdata.usgs.gov/ny/nwis/

inventory/?site_no=04233500&agency_cd=USGS.

Wernly, J.F., and Nystrom, E.A., 2016, Geospatial dataset of 

bathymetric survey of the Cayuga Inlet flood-control channel and selected tributaries, Ithaca, New York, 2016: U.S. 

Geological Survey data release, accessed December 15, 

2016, at https://dx.doi.org/10.5066/F7K935NQ.

Wilson, G.L., and Richards, J.M., 2006, Procedural documentation and accuracy assessment of bathymetric maps and 

area/capacity tables for small reservoirs: U.S. Geological 

Survey Scientific Investigations Report 2006–5208, 24 p.

For more information about this report, contact:

Director, New York Water Science Center

U.S. Geological Survey

30 Brown Road

Ithaca, NY 14850

dc_ny@usgs.gov

(518) 285–5602

or visit our website at

https://ny.water.usgs.gov

Publishing support provided by the Pembroke and Rolla 

Publishing Service Centers
ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20171109

Wernly and others—Bathymetric Survey of the Cayuga Inlet Flood-Control Channel and Selected Tributaries in Ithaca, New York, 2016—Open-File Report 2017–1109