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4.3.4 Method of Navigation and Method of Horizontal Control

In this discussion, navigation refers to real time position feedback to the vessel operator, and horizontal control (survey) is the measured position of the vessel at specified "fix" intervals. Generally, it is desirable for the method of navigation and the method of horizontal control to be the same, but sometimes this is not possible. An example would be a survey with no navigation and with horizontal control using theodolites on the beach, such as a traditional diver survey of an existing cable route. In this case, the theodolites simply track and record the position of the diver's towed buoy.

Fathometer

Sea surface

Fathometer

Sea surface

Fathometer
Figure 4-5 Fathometer footprint.

Methods of horizontal control vary from the simple diver's circling line and compass to Differential GPS. Other common methods are:

• Horizontal sextant angles

• Theodolites or transits

• Theodolites with Electronic DistanceMeters (EDM) (Hydro-I and II)

• Radio Navigation Systems (RNS) (Loran-C network)

• Electronic Ranging Systems (ERS) (Mini-Ranger and Del Norte)

• Acoustic Navigation Systems (ANS) (Diver Navigation System)

Other examples of acoustic navigation systems are Long Baseline systems (LBL), such as SonarDyne's low, medium, and extra high frequency systems and Ultra Short Baseline systems, such as Trackpoint II.

Inmost cases, horizontal control is maintained by measuring, or computing from indirect measurement, the offsets from known control points.

Two corrections need to be made to the horizontal control data:

• A correction for any offset between the control point on the vessel and the location of the depth sensor (fathometer transducer).

• A slant range correction if the horizontal control stations are not at the vertical datum.

Corrections for offset require that the heading of the vessel be known, or approximated, by the course made good. An example of large offset corrections occurs when conducting a side scan survey, where the sensor (towfish) is approximately a distance of one water depth behind the vessel.

Some horizontal control systems also provide some navigation capability but an integrated navigation system such as the HYP AC® software is usually desirable.

The horizontal datum should be selected based on the availability of control points, the equipment being used, and sponsor's requirements. When possible, it is desirable that the horizontal control be in UTM units with the UTM zone noted (when control points are given in geodetic (latitude and longitude), it is desirable to convert to a map projection grid, such as UTM or State Plan Coordinates).

The UTM system presents data in Northing and Easting coordinates with meters as the unit. This is easier for the navigator and helmsman to understand and is easier to plot. When integrated navigation systems are used, it is often possible to convert horizontal datums in real time. When using nonintegrated methods, it is best to work in the same system as the control points and convert datums when the final plots are made. NGS/NOAA provide computer software for conversion between coordinate systems and datums. The program "UTMS" will convert from North American Datum (NAD) 27 or NAD 83 to and from UTM (USGS - phone (301) 713-3242 for more information).

4.3.5 Selecting and Establishing Control Stations

The existing National Ocean Survey (NOS, formerly USGS) control stations should be used whenever possible. Control stations for both theodolite and ERS stations should have line-of-sight coverage of the survey area. For triangulation and trilateralization (transit, theodolite, and most ERS) systems, the stations should be located so that the angle of intersection at the vessel should be between 35 and 135

degrees, to minimize Geometrical Dissolution of Position (GDOP). To meet these requirements, it will frequently be necessary to establish new control stations. Typically, new control stations are created by extending existing control networks using standard land survey procedures. Alternate methods include GRS, Differential Global Positioning System (DGPS), and Kinematic DGPS.

New control stations should be tied to existing survey networks (geodetic control). When this is not possible, they should be tied into local base maps (7.5- and 15-minute quadrangle sheets or Public Works (PW) drawings). Of these methods, the least desirable survey is one which is based on an independent local grid.

The control stations used for hydro-graphic surveys should be Third Order Class II or better, in general. Table 4-3 gives the distance accuracies for various survey classifications.

Table 4-3 Survey Classifications

Distance Accuracy

First Order Second Order, Class I Second Order, Class II Third Order, Class I Third Order, Class II

1:100,000 1: 50,000 1: 20,000 1: 10,000 1: 5,000

For transit and theodolite surveys, two stations are generally adequate. These survey methods use triangulation and die shore stations should be selected so that the included angle, Figure 4-6, is between 35 and 135 degrees for acceptable accuracy. The best accuracy is achieved when the included angle is 90 degrees.

Figure 4-6 Example of layout for conducting survey using triangulation with transits and theodolites.

Figure 4-6 Example of layout for conducting survey using triangulation with transits and theodolites.

Electronic Ranging Systems (ERS) use trilateralization, and in general three or more control stations are desired. The same 35- to 135-degree included angle rule applies to this method when only two stations are used. When more control stations are used, the solution should be based on a least squared error calculation of all possible solutions.

The range and bearing system (Hydro-I and II) requires that only one control station be occupied, but another station is required in order to determine grid north. In the case of Differential GPS, a single station is required.

The success of the survey often depends on the selection of the benchmarks. Information on existing control stations is available from:

NOAA, National Geodetic Survey N/CG17

SSMC3, Station 09202 Silver Springs, MD 20910 Ph: (301) 713-3242 Fax: (301) 713-4172

as either paper copy or ASCII files on computer disk. Benchmarks should be easily relocatable for future survey work and the field notes should provide detailed instructions for finding and identifying the benchmarks used. It is often desirable to set up witness post/marks for the benchmarks used. These could be stakes driven at the side of a road, tags on telephone poles, or paint marks on the road.

If there are insufficient benchmarks in the area of a project, it may be necessary to create one or more. The following methods should be used to establish new benchmarks:

• On rock - Mark the location with a 6-by 6-inch metal plate (stainless steel, brass, or bronze). An engraved X in the center of the plate will mark the exact location of the benchmark. The plate should have the date when it was installed and an identifying name engraved on it, along with the words "For Information Call (202) 433-3881" (NFESC Code ESC55) (recommended naming convention is "UCT-" followed by julian date and sequence letter, i.e., UCT-4151A, UCT-4151B,etc.). The plate may be fastened to the rock using small rock-bolts (3/8 by 6 inches) after suitable holes are drilled in the rock using a hand star drill. An alternative method is to use lead masonry anchors driven into holes drilled into the rock. The plate could then be secured by standard hex bolts. Two remote monuments should be established 10 to 100 feet from the base monument. These should have the names UCT-XXXXA-RM-1 and

UCT-XXXXA-RM-2. The location of the remotes should be such that anyone vandalizing the base or a remote will not see the others.

• On soil - Mark the location with a steel stake driven 3 to 4 feet into the ground. An 8- to 10-inch-diameter concrete pad, 18 inches deep, should be poured around the stake to ensure the position is securely marked. The stake and pad should protrude 2 to 3 inches above the ground level. The exposed steel stake should be filed flat and center punched. A plate with the same information, minus the engraved X, should be bolted to the pad using preset lead masonry anchors.

• On beaches - It is recommended that benchmarks not be set seaward of the dune line and that care be taken in setting benchmarks landward of the dune line because of the instability of sand, which may cover even a permanently placed mark. If it is necessary to set up on the beach, it is recommended that two remotes be established behind the dune line and that the project logs indicate that the base monument was a temporary monument. Any future survey would have to relocate the base station, in the same manner as when a base station is vandalized or destroyed.

After the benchmarks have been physically constructed, their position must be determined from a known benchmark. Known benchmarks to be used, in order of preference, are:

• Permanent Government benchmarks, such as NOS or State grid.

• Local survey markers, when the coordinates of the marker are known, i.e., the center of a manhole cover may be used by local surveyors. Other local survey marks include lot corner stakes and other semipermanent markers.

• Specific portions of a large permanent object (e.g., "2 feet south of southern leg of antenna No. 3") or a specific corner of a building (e.g., "2 feet north or northeast corner of Bldg 1002"). It is prudent to put a marker at these locations. Generally, this will not meet the Third Order Class II requirement, but will meet the permanent object on a base map criteria.

4.3.6 Method of Vertical Control

For a bathymetric survey, vertical control has four parts:

• Datum selection

• Fathometer corrections (speed of sound and draft)

• Tidal corrections.

The overall vertical accuracy is the root mean squared (RMS) of the individual accuracies, assuming they are independent random errors. That is:

Vertical Inaccuracy = Error (RMS) = ± [Sqrt {(Datum error)2 + (speed of sound error)2 + (draft error)2 + (vertical error)2 (tide error)2}]

The selection of a datum is usually driven by the available vertical control points, if they are related to mean sea level (MSL), then that should be the first choice for a datum. It is possible to convert datums, i.e., from MSL to mean lower low water (MLLW) or height above spheroid (for GPS derived data). When doing as-built surveys, the datum should be taken from the construction drawings.

The fathometer correction has two parts: (1) draft correction, and (2) velocity of sound correction. A modern survey fathometer will allow the draft and sound velocity to be entered and thus corrected for by the instrument. The final correction is for tide or the elevation of the water surface above the datum. Since tidal corrections are usually post-processed, it is recommended that both draft and sound velocity be post-processed. In using either method, it is imperative that good field notes be maintained, noting if draft and velocity corrections have been entered and what values have been used. If the recommendation to post-process draft and sound velocity are used, the entered values should be zero for draft and 1,460 m/ sec (or 4,800 ft/see) for sound velocity. Then when the tide correction is made, the data can be corrected using the following equations:

Depth (m) = Velocity (measured)/1,460 (m/ sec) x Depth (m-raw) -I- Draft (m) +Tide (m)

Depth (ft) = Velocity (measured)/4,800 (ft/ sec) x Depth (ft-raw) + Draft (ft) + Tide (ft)

When correcting for tide, make sure that the time of the depth reading corresponds to the time of the tide reading. This is especially critical when using the tide table and local times.

When correcting for sound velocity, make sure that the velocity (measured) is the average vertical velocity, not the surface or bottom velocity.

4.3.7 Data Logging Requirements

The logging, reduction, and data processing are critical in the development of accurate surveys. The logging can take many forms, from the simple handwritten data to a fully integrated system. A desirable bathymetric survey system integrates horizontal control and the fathometer in a computer. In an integrated navigation system the logged data are: fix, time, X,Y,Z, where time is local time (HH:MM:SS) and X, Y, and Z are the coordinates of the fix and the raw (or processed) depth. In this case there is no data reduction and the processing is given by Equation 4-3.

When vertical motion is not automatically corrected or recorded, it is necessary to compare digital information against analog (chart paper) records to make sure that no systematic error has been added by the digital sampling. When interpreting chart records, the interpreter will generally correct for vertical motion (waves) while a digital sampling takes the absolute value of depth, which is depth + wave.

Even when integrated navigation is not available, it is helpful to automatically log the output of a sensor (fathometer or horizontal control system) using a personal computer's serial data (RS-232) input, the serial data output of the sensor, and a communications program.

Data annotation is also critical. The following is the recommended annotation for all chart type data:

At Start of Line (SOL) Survey Area Line Number Date and Time

Start of Line Fix Number (SOL)

Heading

Speed

Initial Instrument Settings (Gain,...)

At End of Line (EOL) Line Number

Date and Time

End of Line Fix Number (EOL)

At Each Fix Fix Number Time

At Any Change of Setting (Gain,...) Old Value New Value

When conducting surveys with towed sensors, it is critical that the offset of the sensors be recorded and taken into account when reducing the data. The position recorded is the position of the navigation antenna, not the position of an offset sensor, i.e., a side scan sonar towfish will be hundreds of feet aft of the vessel at the time of the fix. A highly integrated navigation system may allow you to input the offsets and will track one or more targets. Thus the value for position at a fix will be the true position of the target, not the position of the navigation antenna. When using towed systems, it is critical that the offsets be annotated.

In addition to the record annotation, a logbook should be kept noting the information contained in the annotation plus any operator observations.

Thus far, the emphasis has been on conducting a survey with an integrated navigation system, but at some time a survey may have to be conducted using manual recording techniques. The recommended procedure is to record fixes based on elapsed time, say every 2 minutes. A team of three people would be used, one to monitor time, one to log navigation, and one to annotate the fathometer chart. The annotation should be fix number and time.

4.3.8 Data Reduction

If the fathometer data was recorded on chart paper with fix marks and the navigation system recorded UTM northings and eastings, the data reduction process would create a fix versus depth listing from the fathometer chart which could be merged with the fix versus time-and-coordinate listing from the navigation system to produce a fix, time, X, Y, and Z listing, which could then be processed.

The application of tide corrections can be done with varying degrees of precision; for short lines a single correction is generally appropriate. For long lines or rapidly changing tides a linear approximation with time is generally appropriate, except near high and low tide. A point-by-point correction is the most accurate.

When reducing data, it is necessary to check between fixes, on the chart paper, to determine if the slope is flat or linear (up or down). If the slope is flat or linear, no special processing is required, but if local highs or lows occur, add an intermediate fix and record the high or low.

4.3.9 Data Processing

Once a vertically and horizontally corrected set of data exists, it needs to be contoured. This may be done by hand, on a computer graphics program, on a computer-aided drafting (CAD) program, or using integrated navigation system software such as HYP AC®. The importing of the survey data into a CAD program is desirable, since the final plots can easily be generated by the CAD program.

4.3.10 Deliverable Product

The deliverable product is generally a Public Works drawing of the bottom con tours, with a number assigned by the local Public Works department (see Section 4.5 for more details on producing final drawings). The drawing scale should be ratio metric (1:1,000) rather than engineering (1 " = 1 '). In addition, the scale should be selected to either maximize use of the sheet or to match other drawings or maps. A North-up layout should be used. To maximize the use of the sheet, the maximum North-South (N-S) and East-West (E-W) dimensions of the project area should be determined and converted to inches. The usable height and width of the drawing sheet should be determined in inches.

The ratio of N-S divided by height and E-W divided by width should be determined. The map scale is the largest ratio rounded up to a standard scale. Table 4-4 lists the recommended standard scales. If the survey data is to be used with a set of construction drawings, the scale should be the same as the construction drawing. It is recommended that the map (ratio) scaling convention be used, thus 1 " = 1 ' would be l:12and 1" = 100' would be 1:1,200. Finally, if the data is to be used with existing charts, it should be at the chart scale.

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