Calculate Tools
Overview of Tools
| Icon | Descriptions |
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Calculate the difference between a raster layer and a reference |
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Calculate the volume difference between bathymetry layers or against a reference level within defined polygon areas |
Calculate Raster
The Calculate Raster tool allows calculations to be made, either between two overlapping bathymetry layers, or between one bathymetry layer and a fixed value. This has many useful purposes, including allowing users to compare designs with the reality on the seabed.
When a calculation is made, a new layer is created.
All the basic arithmetic operations (add/subtract/divide/multiply/average) are supported.
Prerequisites:
- At least one bathymetry layer loaded in your project.
- One or more polygon features defining calculation area(s).
The Calculations menu is only available when you have at least one bathymetry layer in your project.
Options
| Option | Description |
|---|---|
| Mode | Layers Makes mathematical calculations between two bathymetry layers Fixed value Makes mathematical calculations between a bathymetry layer and a fixed value |
| Bathymetry layer A | The current situation |
| Bathymetry layer B | If mode = “layers" this is the reference layer/design |
| Fixed value | If mode = “fixed value" then this is the second parameter in the calculation operation |
| Operation | Add A + <layer B/value> Subtract A - <layer B/value> Divide A / <layer B/value> Multiply A * <layer B/value> Average (A + <layer B/value>) / 2 |
| Layer | The name of the layer to be created |
| Personal layer | Only possible for those with a contributor role. Select “Yes" visible to you |
| Layer group | Place the new layer in this layer group |
Instruction Video
Calculate Volume
The Calculate Volume tool calculates the volume difference within defined polygon areas between two bathymetry layers or against a reference level. This is commonly used for monitoring scour protection around monopiles and determining material volume/dredging requirements. The tool outputs into PDF or CSV format.
Quick Start
Prerequisites:
- At least one bathymetry layer loaded in your project.
- One or more polygon features defining calculation area(s).
The Calculations menu is only available when you have at least one bathymetry layer in your project.
Modes:
| Mode | Use When | Example |
| Reference Layer | Comparing two bathymetry surveys to detect changes over time or want to calculate volume to meet requirements after erosion | Comparing pre-installation seabed scan with current conditions around monopile foundation. |
| Reference Level | Calculating volume relative to a fixed elevation |
Calculating rock dump volume from a fixed level to the top of the rock dump. |
Example: reference layer
Example 1 - Design as the reference layer (vs Seabed)
Example 2 - Post Dredge Seabed as reference layer (compared with Pre Dredge Seabed)
Red = Volume above the reference layer
Green = Volume below the reference layer
Example: reference level
Red = Volume above the reference level
Green = Volume below the reference level
Basic workflow:
- Select a model layer (the bathymetry layer to analyze)
- Choose calculation mode (Layers or Reference Level)
- Select reference layer (Layers mode) or enter reference level in meters (Reference Level mode)
- Select polygon layer containing your areas of interest
- Choose one or more polygons to calculate volume for
- Click Export to CSV or Export to PDF
Layer terminology
| Layer | How to use |
| Model Layer | This is the baseline to which you will be comparing a design or DTM (digital terrain model) |
| Reference layer |
This is normally the design that you will be comparing to the model layer (baseline) |
| Reference level |
The depth level in relation to vertical reference that you will compare to the model layer (baseline) |
Understanding Results
How volume is calculated (nearest-neighbour resampling)
Nearest Neighbor is a method used to align two raster layers of different detail levels (resolutions) without changing the original data.
What happens in your project?
When comparing a 1x1m survey to a 0.25x0.25m model, or visa versa the volume calculator needs them to speak the same "language." Using Nearest Neighbor, it divides each large 1m cell into 16 smaller 0.25m cells.
Why use this for volumes?
- Zero Distortion: It doesn't "invent" new heights. It simply duplicates the measured 1x1m height into the smaller squares.
- Mathematical Purity: If your 1x1m square says the ground is at 10 meters, all 16 smaller squares will also be exactly 10 meters.
- Honest Comparison: Other methods (like "Bilinear") would try to smooth the edges, creating "fake" slopes between points. Nearest Neighbor ensures you are comparing your actual measurements directly against the reference model.
The Result
It’s like taking a large Lego brick and swapping it for 16 tiny ones of the same height. The total volume remains identical, but the grid now perfectly matches your high-resolution reference model.
Nearest Neighbor keeps data honest. It changes the 'grid size' without changing the 'measured value,' making it the safest choice for calculating volumes between different survey sources.
The volume calculation produces the following metrics for each selected polygon:
| Metric | Description |
|---|---|
| Model Name | The model layer that is being used to calculate volume for |
| Reference Name | The layer that is being used as the reference for the calculation |
| Polygon Name | The area for which volume will be calculated |
| Volume Above Reference (m³) | Volume where the model layer is higher than the reference (material addition or accumulation) |
| Volume Below Reference (m³) | Volume where the model layer is lower than the reference (material loss or erosion) |
| Volume Difference (m³) | Net volume change (Volume Above minus Volume Below). Positive values indicate net accumulation; negative values indicate net loss |
| Area Above Reference (m²) | Surface area where the model layer is above the reference |
| Area Below Reference (m²) | Surface area where the model layer is below the reference |
| Polygon Area (m²) | The area of the polygon (regardless of whether there is area with or without data) |
| Area without Data (m²) | Area inside the polygon which is not covered by the model layer (required to be able to calculate the average layer thickness) |
| Area without Reference (m²) | Area inside the polygon which is not covered by the reference layer (required to be able to calculate the average layer thickness) |
| Average Layer Thickness (m) |
Average heights of the layer that falls withing the polygon (and coverage with data and reference) |
All values are rounded to two decimal places in the output.
Tips and Best Practices
- Plan your polygon areas carefully: Create polygon layers that match your analysis needs before starting calculations. Well-defined areas of interest will make your workflow more efficient.
- Use meaningful layer names: Name your bathymetry layers with dates or survey identifiers to easily track which data you're comparing.
- Monitor calculation time: Calculations on large areas or high-resolution data may take some time. The tool displays a loading indicator during processing.
- Export both formats: Consider exporting both CSV and PDF formats—CSV for detailed analysis and PDF for documentation and reporting.
- Interpret Volume Difference carefully: Positive values indicate net accumulation; negative values indicate net loss. This metric is particularly useful for quick assessment of material changes.
- Use map highlighting: Selected polygons are highlighted on the map and automatically centered in the view, making it easy to verify you're analyzing the correct areas.
- Keep data organized: Maintain a clear naming convention for your surveys and regularly export calculation results to track changes over time.
Detailed guide
The Calculate Volume tool enables you to calculate volumetric differences in bathymetric data (bathymetric layers) within specified areas (polygons). This functionality is essential for monitoring seafloor changes, particularly for offshore wind farm applications where scour protection monitoring around monopile foundations is critical.
The tool calculates volumes in cubic meters and areas in square meters, providing detailed metrics including volume above, volume below, and net volume difference for each selected polygon.
Prerequisites
Before using the Calculate Volume tool, ensure you have:
- Bathymetry layers: At least one bathymetry layer loaded in your project. To Layers mode, you need at least two compatible bathymetry layers.
- Polygon layer: A feature layer containing polygons that define the areas where you want to calculate volumes.
- Overlapping data: The model layer and reference (layer or polygons) must have overlapping geographic extents.
The Calculations menu is only available when you have at least one bathymetry layer in your project.
Do we need to say the above twice?
Configuration
1. Mode selection
| Mode | When to use? |
| Layers Mode |
Use this mode when you want to compare two bathymetric surveys to calculate changes over time or want to calculate volume to meet requirements after erosion. This is ideal for:
|
| Reference Level Mode |
Use this mode when you want to calculate volume relative to a fixed elevation value. This is useful for:
|
2. Model Layer Selection
The model layer is the primary bathymetry layer you want to analyze. This is typically your most recent survey or the layer representing current conditions.
Select your model layer from the Model layer dropdown menu. Only bathymetry layers that are ready for calculations will be available in this list.
Important: Once you select a model layer, the tool automatically filters the available reference layers and polygon layers to show only those that are compatible (matching projection and overlapping geographic extent).
3. Reference Selection
The reference defines what your model layer will be compared against. The reference selection depends on your chosen mode:
For Layers Mode:
Select a reference layer from the dropdown. This should be your baseline bathymetry survey, typically an earlier survey that you want to compare against.
Only bathymetry layers that have overlapping coverage will appear in this list.
For Reference Level Mode:
Enter a reference level value in meters. This value represents the elevation you want to use as your comparison baseline.
- Positive values: Measure upward from the seabed (e.g., +2.0 meters above the seabed surface)
- Negative values: Measure downward from the seabed (e.g., -10.0 meters below the seabed surface)
Example: For a scour protection mound where you want to calculate the volume from the top down to 10 meters below the peak, you would enter -10.0 as your reference level.
4. Polygon Layer Selection
Select the feature layer that contains the polygons defining your areas of interest. These polygons specify the area for which the volume calculations will be performed.
Only layers containing polygon or closed line string features that overlap with your model layer will be available in the Polygon layer dropdown.
5. Polygon Selection
After selecting a polygon layer, choose which specific polygons you want to include in the calculation:
- Click the Polygons dropdown to see all available polygons from the selected layer
- Select one or more polygons (multi-select is supported)
- Use the Select All option to quickly select all available polygons
Selected polygons will be highlighted on the map, and the map view will automatically zoom to show all selected areas.
do we need to explain what could be the problem if they do not see any polygons (again)? I'm assuming they did not read above.
Exporting the results
The Calculate Volume tool offers two export formats:
CSV Export
Exports calculation results as a comma-separated values file suitable for:
- Import into spreadsheet applications (Excel, Google Sheets)
- Further data analysis and processing
- Integration with other software tools
- Creating custom reports or visualizations
PDF Export
Generates a PDF report with one page per selected polygon, each containing:
- A table with all calculation metrics for that polygon
- A map view centered on the polygon area with bathymetry data
The PDF format is ideal for documentation, presentations, and sharing results with stakeholders who don't need to manipulate the data.
To export:
- Click the main export button to use the currently selected format (default is CSV)
- Or click the dropdown arrow to choose between CSV and PDF formats
- The file will be automatically downloaded with a filename based on your model layer name
Common Use Cases
Scour Protection Monitoring
Scenario: You need to monitor the scour protection around monopile foundations and determine if additional material needs to be placed.
Approach:
- Use Layers mode to compare your recent survey (model layer) against the as-built survey, previous monitoring survey or design (reference layer).
- Select polygons around each monopile location.
- Export results to identify locations with negative volume difference (material loss) that may require remediation.
Rock Dump Volume Estimation
Scenario: You need to estimate the volume of rock material in a scour protection mound using a fixed reference level (-10m).
Approach:
- Use Post survey model layer and a fixed level as reference
- Enter -10.0 as the reference level (10 meters below the surface)
- Select the polygon encompassing the rock dump area
- The Volume Above value will represent the amount of material to dump
Change Detection Analysis
Scenario: You need to assess changes across multiple locations between two survey periods.
Approach:
- Use Layers mode with your newer survey as the model layer and older survey as the reference layer
- Select all polygons of interest using the Select All option
- Export to CSV for analysis in spreadsheet software
- Sort by Volume Difference to quickly identify areas with the greatest changes
Troubleshooting
Layer options are limited after selecting model layer
This is expected behavior. The tool automatically filters available layers to show only those that are compatible with your selected model layer (same projection for reference layers, overlapping extent for polygon layers).
Error: Bathymetry layer(s) are missing the required data file
This error occurs when one or more selected bathymetry layers don't have the necessary processed data for calculations. Ensure that your bathymetry layers have been fully processed before attempting volume calculations.
Unexpected or incorrect results
If results seem incorrect:
- Verify that both layers (in Layers mode) use the same coordinate reference system
- Confirm that your polygons actually overlap the bathymetry data coverage area
- Check the vertical reference system of your bathymetry layers
- In Reference Level mode, verify you've entered the reference level with the correct sign (positive or negative)