SonarWiz Gridding Options · SonarWiz_GriddingOptions.PDF Chesapeake Technology, Inc. copyright...

SonarWiz Gridding Options

CTI Technical Note

Revision 9.0, 10/22/2018

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SonarWiz_GriddingOptions.PDF Chesapeake Technology, Inc. copyright 2016-2017

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Table of Contents 1 Gridding Data in SonarWiz ....................................................................................... 4

1.1 TOOLS -> Grid/Contour Grid Options (SonarWiz 5.08, 6, 7) ............................. 4

1.1.1 Mag data - generating grid / contour lines from CMF files in Project ........... 5 1.1.2 Bathymetry - Exporting an XYZ dataset, then using Tools -> Grid/Contour 7

1.2 SB Reflector Gridding Options (Available in SonarWiz versions 5.08, 6, 7) ..... 10

1.2.1 SB reflector export as 3D surface - reflector naming ................................. 10

1.2.2 SB reflector export as 3D surface - EXPORT dialog example ................... 12 1.2.3 SB reflector export as 3D surface - SHP folder results .............................. 12 1.2.4 SB reflector export as 3D surface - Grids -> Add/Import Grid .................... 12 1.2.5 SB reflector export as 3D surface - Color Window Adjustments ................ 14 1.2.6 Exporting sediment layer reflectors to a 3D surface .................................. 17

1.3 Bathymetry Gridding Options (Only available in SonarWiz 6,7) ....................... 18

1.3.1 INTERNAL Grid Generation ...................................................................... 20 1.3.2 EXTERNAL XYZ Grid Generation ............................................................. 21

2 SonarWiz 6 INTERNAL Project Data - Gridding Options ....................................... 22

2.1 Project Data - Amplitude Gridding Options ...................................................... 22

2.1.1 Amplitude Gridding - Example Dialog- Algorithm Choices ......................... 23

2.1.2 Amplitude Gridding - Algorithm Descriptions ............................................. 23 2.2 Project Data - CSF SS Gridding Options ......................................................... 24

2.2.1 CSF SS Gridding - Example Dialog - Algorithm Choices ........................... 24 2.2.2 CSF SS Gridding - Algorithm Descriptions ................................................ 25

2.2.3 CSF SS Gridding - Seeing it Scaled to Color - as BACKSCATTER .......... 25

2.2.4 CSF SS Gridding - starting with small file-count or smaller files ............... 25 2.3 Project Data - Depth Gridding options .............................................................. 26

2.3.1 Depth Gridding - Example Dialog .............................................................. 26 2.3.2 Depth Gridding - Algorithm Descriptions.................................................... 26

3 EXTERNAL XYZ Data - Gridding Options .............................................................. 28

3.1.1 External XYZ Gridding - Algorithm Descriptions ........................................ 29

4 Grid Options - Common to All Grid Types .............................................................. 29

4.1 Cell Size Control .............................................................................................. 29

4.2 Grid Extent Controls ......................................................................................... 30

4.3 Output Grid File Root Name ............................................................................. 30

5 Grid Mathematics - Formula Names Explained ...................................................... 30

5.1 Blend (Advanced Option Controls - Amplitude Blend) ...................................... 30

5.2 CUBE (Depth only) ........................................................................................... 30

5.3 Depth Blend (Depth only) ................................................................................. 31

5.4 Inverse Distance Weighted .............................................................................. 32



5.5 Mean ................................................................................................................ 32

5.6 Median ............................................................................................................. 32

5.7 Median (Advanced Option - Amplitude Median) ............................................... 33

5.8 Natural Neighbor (External XYZ and Internal DEPTH data only) ..................... 33

5.9 Nearest Neighbor (External XYZ data and Internal DEPTH data only) ............ 36

5.10 Nearest Range (Amplitude, CSF sidescan only) ........................................... 36

5.11 Preferred Angle Weighted (Amplitude, CSF sidescan only) .......................... 36

5.12 Trimmed Mean (Depth only) ......................................................................... 38

6 SonarWiz Grid Support for Dredging & Shore Geomorphology ............................. 38

6.1 Example Applications - Dredging and Shore Geomorphology Studies ............ 38

6.2 Opening the Grid Volume Calculator ................................................................ 39

6.3 Volume Calculator Options............................................................................... 39

6.4 Volume Calculator Outputs .............................................................................. 40

6.4.1 GRID Output Example ............................................................................... 40 6.5 Incidental benefit - GRID Inversion .................................................................. 42

6.5.1 Setting up Volume Calculator for GRD inversion ....................................... 43

6.5.2 Viewing your Inverted GRD File ................................................................ 44 7 Document Revision History .................................................................................... 45



1 Gridding Data in SonarWiz

This document explains some of the many ways in which gridded data may be

generated by SonarWiz. The newer options were introduced in SonarWiz (6 and 7), but

are not available in SonarWiz 5. Gridding is a mathematical re-presentation of spatial

data, like backscatter or sidescan (amplitude), bathymetry (depth), or magnetometry

amplitude, in which the regular tessellation (tiling) of a 2D or 3D surface is smoothed

according to a variety of mathematical formula options. It can sometimes give a visual

effect of appearing better than the original data set (point cloud), and in this way, can

enhance modeling of the underlying real surface, and enhance interpretation. Much of

the mathematics of the various gridding options presented in SonarWiz, may be

understood more deeply, by researching each gridding technique on the Internet. We

will give at least a superficial, if not mathematically rigorous, explanation of each

SonarWiz gridding technique, in this technical note.

1.1 TOOLS -> Grid/Contour Grid Options (SonarWiz 5.08, 6, 7)

Historically, SonarWiz evolved an early-model gridding option in the TOOLS ->

Grid/Contour section of the GUI, which is still present. This function is available in all

version of SonarWiz, and is well-described in the SonarWiz USER GUIDE, and will not

be explained in detail here. It supports our early work in grid and isopach-type contour-

line representation of internal project data, or external XYZ data, and is still an excellent

source of grids for data types such as sidescan or sub-bottom altitude+sensor-depth

data or magnetometry data, from the project.

This grid/contour function can be used with any post-processing license in SonarWiz.



1.1.1 Mag data - generating grid / contour lines from CMF files in Project

Output of magnetometry data to ESRI shapefile, for example:

creates a SHP file contour-lines export (add to project as a basemap) as well as a GRD

file (add to project using Grids -> ADD GRID). "Variable" is only active when you select

Data Source = Use Data from Project. In this example, the Variable = Magnetometry

and Data Source = Use Data from Project allows us to select (lower in dialog, not

shown) specific CMF files in the project to use for the data to be gridded/contoured. You

can specify a subset, not necessarily all the CMF lines!

Example: Magnetometry contour line SHP file generated this way:



Corresponding GRD file created in the same operation:

NOTE: Choosing "Bathymetry" in the main color window works well for SonarWiz 6 and

SOnarWiz 7 only up to 7.01 series. For 7.01.004, 7.02 and subsequent versions of

SonarWiz, there are separate group color (main color window icon) and solo-item (item

selectable in Project Explorer). Mag data is not part of any group in 7.01.004 and 7.02

series SonarWiz (i.e. it is not subject to "bathymetry" group color management), so color

management requires that you select your grid item in the project explorer, and then

select PROPERTIES -> DISPLAY and de-select the "use global color group" check-box,

then select the colo-window line-item in PROPERTIES-> DISPLAY and select the

ellipsis icon, to open the solo-item color management window.



Then you can color-scale the mag data just like you could with the main color window in

SonarWIz 6 and SonarwIz 7.01. See the DOCS folder of your SonarWiz 7.02

installation (Use HELP -> Browse HELPDOCS folder) and open the Introduction to

SOnarWiz 7 and look in sections 6.2 and 6.3 for a more complete example of the group

vs solo-item color window management techniques.

1.1.2 Bathymetry - Exporting an XYZ dataset, then using Tools -> Grid/Contour

Here's another example. You really have 3 options when gridding bathymetry data.

(1) Use the GRIDS -> Create GRID function (described below) and grid selected files

from the project

(2) Use the GRIDS -> Create GRID function and select to use an External XYZ data set.

or

(3) Use the Tools -> Grid/Contour option and grid your bathymetry data represented as

an ASCII XYZ dataset )CSV file).

Each technique has its pros and cons. Options (1) and (2) require 2 steps, since the

bathymetry CREATE GRID option grids the data first, then in a second step you can

create contour lines. Some people prefer the legacy gridder option (3), because the

integral grid/contour dialog has been around a long time, wortks great, and creates both

the contour lines SHP file, and the Surfer6-format GRD file, in the same single

operation. Both export to your SHP folder and a companion PRJ file is created to help

importing the GRD later.



If you want to use options (2) or (3) with bathymetry data, once you have imported and

merged your bathymetry data set, export it to an XYZ ASCII CSV (comma-separated

value) format text file this way:

That leads to this dialog next:

An export of a single bathy file can generate quite a large-sized ASCII CSV format XYZ

file result, and note that it expoerts to the XYZ fodler of your project:



Then this XYZ file can be used as input to the Tools -> Grid/Contour function like this:

If the results are not what you want, try option (1) ... it certainly has its merits! Details of

that gridding technique are explained below.

Please see the NOTE section after magnetometry example above, since the grid data

imported this way may also be best color-mapped using the solo-item color

management techniques in 7.01.004 and 7.02 series.



1.2 SB Reflector Gridding Options (Available in SonarWiz versions 5.08, 6, 7)

There is a wonderful sub-bottom reflector export function for creating GRD files too, and

we will detail that in this section below. Basically, the idea is to capture a reflector which

is the same in a set on sub-bottom files in the project, give all those reflectors the same

name, then export them to a 3D surface (Surfer6 GRID file - e.g. *.GRD). The GRD file

can then be viewed within the project using ADD GRID, or viewed in the CTI3DViewer

in 3D format. Here is an example project with 32 seafloor reflectors showing, where

bottom-track was saved to a reflector using MAKE REFLECTOR for each file:

1.2.1 SB reflector export as 3D surface - reflector naming

Name your reflectors the same name to easily export them to a 3D surface. In the

Feature Manager, you can rename them if, for example, seafloor reflectors had all been

captured, but still have the line-name in the feature name. In this example, we renamed

all the seafloor features to "S1":



The results:

Next step is to choose EXPORT.



1.2.2 SB reflector export as 3D surface - EXPORT dialog example

These are the 3 selections to make to do this export of all reflectors names "S1" to a 3D

surface.

1.2.3 SB reflector export as 3D surface - SHP folder results

The exports are a set of files going to the SHP folder of your project:

The GRD file will be specified in the next step when you do Grids -> ADD/IMPORT

GRID.

1.2.4 SB reflector export as 3D surface - Grids -> Add/Import Grid

Right-click on "Grids" in the left-side project explorer:



Then select Add/Import Grid Files, and browse to your GRD file. To finish the import,

click OK if you are prompted to confirm the coordinate system of the grid:



1.2.5 SB reflector export as 3D surface - Color Window Adjustments

The key to grid display is to select to open and dock a COLOR WINDOW. This is all

explained in more detail here:

http://www.chestech-support.com/download/ctisupport/Sonarwiz_6/UserDocs/Sub-

bottomImport_6.04_6.05_Advice.pdf

http://www.chestech-support.com/download/ctisupport/Sonarwiz_6/UserDocs/Sub-bottomImport_6.04_6.05_Advice.pdf

http://www.chestech-support.com/download/ctisupport/Sonarwiz_6/UserDocs/Sub-bottomImport_6.04_6.05_Advice.pdf



1. Open and dock your COLOR WINDOW

2. Select Bathymetry data type, for any grid you need to view

3. Select a color palette, then click on SCALE TO DATA.

Here is an example grid names S1.GRD from the data shown above, using

DefaultColorMap 3D color palette:



If you have simply bottom-tracked and not accounted for sensor depth, your grid may

incorporate only altitude data (sensor altitude above seafloor). To have it incorporate

sensor depth, be sure the SENSOR DEPTH property of all files is non-zero and maybe

set sensor depth before doing the seafloor capture (bottom-tracking), or gridding steps.

You can set sensor depth in the Sonar File Manager here:



1.2.6 Exporting sediment layer reflectors to a 3D surface

The 3D seafloor surface grid is a common one to export. Less frequently, one may want

to capture a 3D surface of a sediment layer. This can be done in the same project, by

using these steps:

1. Capture reflector "R1" identically named, in a set of files. You capture it separately in

each file, but be sure each reflector represent the same sediment layer, and has the

same name "R1" (just my example name).

2. Then follow the same steps above, but select "R1" feature as the name to export to a

3D surface.



1.3 Bathymetry Gridding Options (Only available in SonarWiz 6,7)

Also available with any post-processing license in SonarWiz 6 or 7, is the newer

bathymetry gridding dialog. Access the newer gridding dialog by right-clicking GRIDS in

the ProjectExplorer,

and selecting Create New Grid... from the drop menu. It supports INTERNAL gridding,

or EXTERNAL gridding.

When you select that, the main gridding dialog opens:



Grids are able to be viewed a variety of ways (e.g. main map view, or CTI3DViewer, or

various editors). Each grid file created, is saved in the project itself in the GRIDS folder,

in a file named by default with a GRD extension, like this, often with companion files of

PRJ and XML type:



Companion files help SonarWiz manage viewing and use of the GRD file itself.

1.3.1 INTERNAL Grid Generation

INTERNAL gridding is using existing data from within the specific SonarWiz project,

which you have open, to produce a modified representation of the project data in a grid.

As of SonarWiz release 6.05.0002, these types of internal project data may be gridded

using the main gridding dialog: Amplitude, CSF sidescan, and Depth.



Amplitude gridding - This is the backscatter component of a bathymetry post-

processing project. You create a project and vessel offsets definition first, then import

and post-process the backscatter data first, to a point where it will support gridding, then

can use this internal project data to create grids. Backscatter of currently enabled

bathymetry lines in the map view are gridded. Choose backscatter as the data type in

your color legend.

CSF sidescan - This is a rough equivalent to backscatter, but grids the amplitude data

from your sidescan project, whichever sidescan lines in the project are currently

enabled for view in the map view. Sidescan data is of amplitude type, but is not as

exactly located in 3D space as the backscatter component of multibeam echosounder

(MBES) or interferometer survey results. Choose backscatter as the data type in your

color legend.

Depth - This is the bathymetry component of your MBES or interferometer survey data,

used as input to the gridding math to produce richly diverse, smoother, and in sow ways

better representations of your X,Y,Z (depth) point-cloud data. You create a project and

vessel offsets definition first, then import the bathymetry data files, merge them and

perhaps edit/filter and adjust, then grid the data. Depth data of currently enabled

bathymetry lines in the map view are gridded. Choose bathymetry as the data type in

your color legend.

1.3.2 EXTERNAL XYZ Grid Generation

This is a newer gridding dialog option for bringing in XYZ data (e.g. a text-format CSV

file - comma- separated-values like this, as viewed in WORDPAD):

The 3 comma-separated columns should be arranged like that, left-to-right, in each line

(row) of the file:

1 - X (e.g., Easting) - left-most column



2 - Y (e.g. Northing) - center column

3 - Z (e.g. depth) - right-most column.

Note that Z is a wonderfully diverse option of your choice. The example says "depth",

but really it's just a number to be gridded, and can be magnetometry amplitude, or

cable-out ... whatever you choose. For best accuracy, and rendition in grids, we

suggest at least 1 decimal point of resolution in the X and Y floating-point values, and

non-integer data in column 3, since floating-point data with higher resolution than

integer data will grid better.

It makes sense to have the coordinate system in which X,Y were recorded, be the same

as that in the project which you are using, and also to have the grid in the same units

(e.g. meters or feet) as the project.

2 SonarWiz 6 INTERNAL Project Data - Gridding Options

This section explains the mathematical options, and "gridding algorithm" names, which

may be chosen to grid the data of each type in the internal project data - amplitude,

CSF sidescan, and depth data.

2.1 Project Data - Amplitude Gridding Options

This type of gridding shows you bathymetry backscatter AMPLITUDE grid data in a

color-coded display.



2.1.1 Amplitude Gridding - Example Dialog- Algorithm Choices

2.1.2 Amplitude Gridding - Algorithm Descriptions

Algorithm Name Description

Mean The mean backscatter amplitude value of all soundings within the cell

Median The median backscatter amplitude value of all soundings within the cell

Blend Smoothly blends backscatter amplitude from overlapping lines.

Nearest Range The median backscatter amplitude from only the soundings within the cell, from the nearest trackline

Inverse Distance Weighted

Inverse-distance algorithm uses advanced mathematics to create an exponential-type function of weighting backscatter amplitude from surrounding nodes within the cell, in the point-cloud. You can adjust this gridding function using ADVANCED OPTIONS to weight the closer, or the farther nodes more.

Preferred Angle Weighted

A backscatter amplitude gridding option where you can give priority to a preferred angle, such as for seeing the better view from one angle or another, where a tall contact creates excessive shadow from a non-preferred angle



2.2 Project Data - CSF SS Gridding Options

This type of gridding shows you CSF sidescan AMPLITUDE data grid data in a color-

coded display.

Example - blend algorithm grid of CSF sidescan data:

2.2.1 CSF SS Gridding - Example Dialog - Algorithm Choices



2.2.2 CSF SS Gridding - Algorithm Descriptions


Mean The mean CSF sidescan amplitude value of all soundings within the cell

Median The median CSF sidescan amplitude value of all soundings within the cell

Blend Smoothly blends CSF sidescan amplitude from overlapping lines

Nearest Range The median CSF sidescan amplitude from only the soundings within the cell, from the nearest trackline


Inverse-distance algorithm uses advanced mathematics to create an exponential-type function of weighting CSF sidescan amplitude from surrounding nodes within the cell, in the point-cloud. You can adjust this gridding function using ADVANCED OPTIONS to weight the closer, or the farther nodes more.

Preferred Angle Weighted

A CSF sidescan amplitude gridding option where you can give priority to a preferred angle, such as for seeing the better view from one angle or another, where a tall contact creates excessive shadow from a non-preferred angle

2.2.3 CSF SS Gridding - Seeing it Scaled to Color - as BACKSCATTER

CSF amplitude gridding can add the grid you your project, but when it does, the grid has

BACKSCATTER datatype. To color-scale your new CSF amplitude grid, select

BACKACATTER data format, then color palette:

2.2.4 CSF SS Gridding - starting with small file-count or smaller files

The computations are memory-intensive! Use a small group of CSF files to start, or grid

a single file, to get the feel of it. The CSF files enabled to be displayed, in the main

map view, are the ones which will be gridded - start small-batch, then try larger



batches, to avoid crashes. Another way to reduce gridding time and increase file-count

in the grid - use 1024 samples/channel option in the SS import, not 2048 or 4096.

2.3 Project Data - Depth Gridding options

This type of gridding shows you bathymetry DEPTH data grid data in a color-coded

display.

2.3.1 Depth Gridding - Example Dialog

2.3.2 Depth Gridding - Algorithm Descriptions


Trimmed Mean The arithmetic mean bathymetric depth, computed within a cell, and within limits selected by you in advanced options

Median The median bathymetric depth value of all soundings within the cell

CUBE Combined Uncertainty Bathymetric Estimator type bathymetry depth grid (see more description in section 5.2 below)


Inverse-distance algorithm uses advanced mathematics to create an exponential-type function of weighting bathymetric depth from surrounding nodes within the cell, in the point-cloud. You can adjust this gridding function using ADVANCED OPTIONS to weight the closer, or the farther nodes more.

Depth Blend Smoothly blends bathymetric depth from overlapping lines

Natural Neighbor A gridding technique which interpolates between possibly multiple nearby adjacent neighbors within the cell, for a best estimated (interpolated) value

Example:

(1) 4 bathymetry lines imported, viewed:



(2) Median depth grid - 2D map view, Tampa Bay, FL barges data set (data available

from www.chestech-support.com Sample Data area):

Example: Same median depth bathymetry grid exported to CTI3DViewer:



3 EXTERNAL XYZ Data - Gridding Options

When you select the EXTERNAL XYZ data gridding option, these are the options for

your choice of the mathematical algorithm to be used to generate your grid:



3.1.1 External XYZ Gridding - Algorithm Descriptions


Mean The grid result is the mean value of all soundings within the cell

Median The grid result is the median value of all soundings within the cell


A gridding technique which allows you to control whether near or far neighbors are weighted more, within the cell

Natural Neighbor A gridding technique which interpolates between possibly multiple nearby adjacent neighbors within the cell, for a best estimated (interpolated) value

Nearest Neighbor A gridding technique which uses the nearest real neighbor within the cell, for a best-choice value

4 Grid Options - Common to All Grid Types

4.1 Cell Size Control

The cell is a space measured in meters distance, e.g. 4 = 4m square space area, within

which point-cloud values will be included in the computation for that cell. Smaller values

here mean higher resolution, and more math computation cycles, so it may take longer.



4.2 Grid Extent Controls

This limits the specific area of the point cloud space, which will be included in the grid

computation.

4.3 Output Grid File Root Name

The grid files will be written to the project GRIDS folder, in a name you choose. The root

name is the name portion, and a specific extension like GRD, XML, PRJ etc will be

added.

5 Grid Mathematics - Formula Names Explained

The section below explains the mathematical process used to generate the grids of

each type named in the "grid algorithm" list of the grid dialog, and in the advanced

options dialog. They are presented below in alphabetic order.

5.1 Blend (Advanced Option Controls - Amplitude Blend)

This algorithm smoothly blends amplitude from overlapping lines. It can be used for

backscatter amplitude, or CSF sidescan amplitude gridding.

5.2 CUBE (Depth only)

This is a bathymetry grid type, called Combined Uncertainty and Bathymetric Estimator

(CUBE), developed by Brian Calder of the University of New Hampshire (UNH) CCOM



program. Their web-site http://ccom.unh.edu/theme/data-processing/cube describes it

like this:

"The technique, known as CUBE (Combined Uncertainty and Bathymetric Estimator), is

an error-model based, direct DTM (ed note: digital terrain model) generator that

estimates the depth plus a confidence interval directly on each node point of a

bathymetric grid."

Here are the options you see when selecting CUBE grid output:

Basically you get an estimate of how accurate your grid data model the underlying

seafloor, in the form of an error-model with uncertainty estimates.

5.3 Depth Blend (Depth only)

This algorithm smoothly blends bathymetric depth from overlapping lines. It can only be

used for bathymetry depth gridding.

http://ccom.unh.edu/theme/data-processing/cube



5.4 Inverse Distance Weighted

This option provides user-control to support weighting the closer, or farther nodes,

within the cell, more.

5.5 Mean

This option computes a grid value using the arithmetic mean value among the nodes

within the cell size.

5.6 Median

This option computes a grid value using the statistical median value (value where half

are above, and half below it, in the node population) among the nodes within the cell

size.



5.7 Median (Advanced Option - Amplitude Median)

For amplitude median (for backscatter or CSF sidescan data), you can limit the

maximum size to grow the sounding buffers per node, and a smaller number uses less

memory. Here is the Advanced Option control for it:

5.8 Natural Neighbor (External XYZ and Internal DEPTH data only)

Wikipedia describes this as an algorithm for spatial interpolation, and is a specific

technique developed by mathematician Robin Sibson. It uses Voronoi tessellation

(tiling) with a mathematical rule defining "natural" adjacent nodes in XYZ space. By

using multiple surrounding, adjacent neighbors in the point-cloud, bordering your central

node, this algorithm takes more time, but can result in a smoother interpolation result,

than nearest neighbor algorithm. If you have the time to render your external XYZ

point-cloud into a grid this way, it may be a better overall appearance, than a nearest

neighbor grid.

You have no controls to adjust, in Advanced Options, when selecting natural neighbor

algorithm.

See: https://en.wikipedia.org/wiki/Natural_neighbor

Natural Neighbor gridding capability has particular application in magnetometry data. The

"Natural Neighbor" algorithm choice works especially well with sparse data like

magnetometry.

Mag data can be exported from SonarWiz in XYZ format, then imported as an external XYZ

data set to make a grid, using Natural Neighbor algorithm. Here's a contour-lines format of

the original mag data:

https://en.wikipedia.org/wiki/Natural_neighbor



And here is the same data, gridded using external XYZ option in the bathymetry

gridding dialog, using Natural Neighbor algorithm:



For a second example, we have a bathymetry point-cloud map view with clear gaps in

the depth data at the sides, and especially at nadir:

A 1-m resolution natural nadir depth grid of this makes a great gap-filling interpolation of

the data:

The results are excellent:



5.9 Nearest Neighbor (External XYZ data and Internal DEPTH data only)

This is a spatial interpolation algorithm which selects the single nearest real XYZ point-

cloud datum as the "neighbor" when creating the grid result, for each grid node. As a

technique it is very fast, and simple, so it has those advantages. If you have a really

large XYZ point-cloud to process, getting an initial impression using nearest neighbor

may be your best option.

See: https://en.wikipedia.org/wiki/Nearest-neighbor_interpolation

5.10 Nearest Range (Amplitude, CSF sidescan only)

This algorithm is like nearest neighbor in XYZ gridding, but looks for the real nearest

slant-range datum in the backscatter or CSF sidescan data, to use as your grid datum.

5.11 Preferred Angle Weighted (Amplitude, CSF sidescan only)

Here are the Advanced Gridding options for preferred-angle weighted gridding

algorithm:

https://en.wikipedia.org/wiki/Nearest-neighbor_interpolation



5.12 Trimmed Mean (Depth only)

The option supports outlier rejection and shoal-ness rejection criteria too:

6 SonarWiz Grid Support for Dredging & Shore Geomorphology

SonarWiz has a very convenient way to compare "before" and "after" seafloor surface

depths, in a way which supports dredging and geomorphology shore studies perfectly.

The feature is available by right-clicking a specific grid in your project in the Project

Explorer, and the choice to make is "volume computation".

6.1 Example Applications - Dredging and Shore Geomorphology Studies

Imagine first, that you have two sets of survey data - such as:

1 - Before a dredging operation

2 - After a period of dredging

Each set of survey data can be used to produce a seafloor surface model GRD file, then

SonarWiz lets you compare these GRD files from each survey, and subtract one from

the other, computing the volume of the difference in cubic meters, for example.

Another dredging application is computing, after only an initial survey, the volume or

material above a certain depth in a channel, which must be removed by dredging.

In shore geomorphology, your two surveys may be months apart, but a comparison

between them is necessary. What is the volume difference between the two surveys?



Has sub-surface shore eroded, or has sediment accumulated, in between the two

surveys?

SonarWiz makes it easy to compare the survey results in each case, with grid volume

calculator support.

6.2 Opening the Grid Volume Calculator

You open the grid volume calculator dialog with a right-click on any grid in the Project

Explorer, then select Volume Calculator ... :

6.3 Volume Calculator Options

You have 3 main options for grid comparison and volume calculation in this dialog:

Option (1) Subtract 2 grids from one another, resulting in a third grid



Option (2) Subtract a depth level from a grid, resulting in a second grid

Option (3) Subtract a grid from a depth level, resulting in a second grid.

The dialog presents like this:

You can see this is a dredging application - where you need to estimate the initial

volume of material ABOVE the required 8m depth in a channel . This would be the

amount of material you need to remove by dredging. You can estimate work time and

costs when you know the depth and volume of material to be removed.

6.4 Volume Calculator Outputs

6.4.1 GRID Output Example

Here is a 2D view of the starting grid DEPMed_DEP.grd, and we have computed the

volume of the difference - between this grid, and the 8m depth level:



The difference grid output is: "Dredging_volume_above_8m_depth.grd" saved into the

same GRD folder of the project:

The companion grid report TXT file export results provides VOLUME, and data from

which you can derive SURFACE AREA:



And with data in the report, you can get a more exact surface area too, multiplying

(cell area) x the (number of filled cells) in the grid = surface area:

Surface Area = 190010 (filled nodes) * 0.063 m^2 (cell area) = 11970.63 m^2

(More exact than our 12000 m^2 area estimate.)

NOTE: Demo is using 6.05.0017, but a subsequent version of SonarWiz volume

calculator will compute the surface area for you - no manual formulas needed.

6.5 Incidental benefit - GRID Inversion

Volume Calculator can also help you if for any reason you need to invert a grid.



If you are doing an operation where depths need to be larger-positive-numbers means

"deeper", like SonarWiz uses grids in datum align operation, and you happen to have a

negative grid (e.g. sea surface = 0.0 but 100m deep = -100.00), then volume calculator

will really help! Just subtract your grid from the 0.00 level and your resulting output grid

will be an inverted "negative" grid, which is a positive grid.

E.g. 0.0 - (-100.00) = 100.0

This works great if your original grid datum was sea-level = 0.00.

Here's an existing GRD in the project where viewing it in COLOR WINDOW and using

SCALE TO DATA shows its depth-range as 4.26 to 13.27 m.

It's already a good GRD to use in the project, but just for an example, we will invert it.

As it is, blue is deep, yellow is shallow.

6.5.1 Setting up Volume Calculator for GRD inversion

We will use depth level 0.00 for the inversion point, to create a grid with the range -4.26

to -13.27 depth values:



The basic operation: LEVEL 0.00m - GRID = Inverted GRID

This works the same for positive or negative grids. Whatever the polarity, it will be

switched in the resulting inverted grid output GRD file.

6.5.2 Viewing your Inverted GRD File

Bring the GRD file into the project and view it as bathymetry data type in a color

window, and click SCALE TO DATA to verify that you now have an inverted grid:



Now yellow is deeper, blue is shallow.

7 Document Revision History

Rev 9, 10/22/2018 Section 1.1 was expanded to describe mag and bathy grid/contour

solo-item color-management options in more detail, since 7.01.004 and 7.02 do not

include mag and external XYZ data in "bathymetry" category.

Rev 8, 9/10/2017 Section 1.1 was expanded to describe mag and bathy grid/contour

options in more detail.

Rev 7, 8/18/2017 New section 6.5 gives a specific grid inversion example.

Rev 6, 6/22/2017 New description section 6 explains volume computation options.

Rev 5, 4/21/2017 News about the recent 6.05.0009 addition of natural neighbor

algorithm option for bathymetry depth data added.

Rev 1, 11/09/2016 Initial draft Rev 1 - Introductory PDF on special topic created

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