Layers

This chapter covers the layers system, how they are created, what they do, how they are visualised and the various types of layers.

Layers provide a way to connect the various processing capabilities of GPlates to data sources (such as loaded feature collections). The outputs of these layers can then be visualised directly in the globe and map views and/or passed to the input of other layers for further processing.

A file contains a collection of features (a feature collection).

A layer processes one or more inputs into an output. Inputs to a layer can include, but are not necessarily restricted to, feature collections. For example some types of layers, such as the Reconstructed Geometries layer, accept both feature collections and the output of another layer.

In the case of the Reconstructed Geometries layer:

The reason the rotations come from the output of another layer rather than a feature collection (containing rotation features) is because a rotation hierarchy needs to be generated from the rotation features themselves and so this process is performed by a different type of layer (the Reconstruction Tree layer). See the More on Reconstructions chapter for more information about rotation hierarchies.

The output of most types of layers (exceptions include Reconstruction Tree layers) contain geometries and hence can be visualised in the globe and map views.

The Layers dialog is usually displayed automatically when you first load a feature collection. To show/hide the dialog, select the Show Layers menu item in the Window menu or use the Ctrl+L shortcut key.

The Layers dialog contains all layers and is the central place to configure layer visibility, draw order, input connections and layer-specific options.

The collapsed view of each layer in the dialog shows a layer name, type and colour. The type and colour are associated (for example, a green layer is always of type Reconstructed Geometries). The layer name depends on how the layer was created (see the Creating layers section for more details).

The visibility of each layer can be individually disabled (or enabled) by clicking the icon to the left of the layer name.

Each layer contains a small black arrow that can be clicked on to expand the layer and show the input connections and any layer-specific options. Once expanded you can click on the symbol to collapse the layer again.

Every layer has an "Input channels" section that displays the current inputs and also allows the user add, remove or change inputs to each layer. Each layer type can have different types of input channels. In the Reconstructed Geometries example above there are two types of input channel, one labelled "Reconstructable features" and the other labelled "Reconstruction tree". The types of input channel are specific to each layer type and will be covered in greater detail in the Types of layers section.

Input connections can be:

In the "Manage layer" section of each layer you can Enable and Disable the layer.

The "Disable layer" and "Enable layer" options determine if a layer does any processing or not. If a layer is disabled then that layer is effectively switched off and nothing is generated or output by that layer. It also means nothing will be drawn in the globe and map views for that layer (regardless of that layer’s visibility). And it means any other layer receiving input from that layer will receive nothing.

Layers are drawn onto the globe and map views in the order in which they are displayed in the Layers dialog. Layers at the top are drawn on top of layers below them.

To change the visual ordering of a layer simply drag it onto another layer.

In this example, the raster layers are at the bottom and hence are drawn underneath the other layers. And the user has selected only one raster to be visible (the visibility icon is on for only one raster layer).

Previous versions of GPlates required layers containing vector geometries to be drawn on top of any raster layers otherwise the raster layers would obscure them (especially if they were global rasters). However GPlates now supports adjusting raster transparency (and intensity) individually per raster layer - see Reconstructed Raster Layer for more details. The following image shows a raster layer (with opacity set to 0.64) on top of layer containing coastlines - the coastlines are under the raster but are partially visible through it.

There are two ways in which a layer can be created. Either automatically by GPlates when the user loads a feature collection or explicitly when the user creates a new layer.

When you load a feature collection usually one (or more) layers are created.

Loading these feature collections…

…will result in these layers being automatically created (in this case one layer per feature collection)…

In some situations loading one feature collection can create two layers.

Here one feature collection containing both Topological Closed Plate Polygon features and the regular features referenced by them is loaded…

…and two layers are automatically created…

One layer reconstructs the regular features that are referenced by the plate polygons and the other layer does the work of stitching the features together, intersecting them and creating the dynamic polygon boundary.

Layers can be explicitly created by the user.

After selecting Add new layer… at the top of the Layers dialog you can then select the type of layer you want to create. Here is example of creating a new Calculated Velocity Fields layer.

A new layer is then created and inserted at the top of the layer stack.

There are various types of layers each represented by a different colour in the Layers dialog.

Each layer provides a different type of functionality, has different types of inputs and generates different outputs.

This layer combines rotation features from one or more feature collections to form a reconstruction tree or rotation hierarchy (see the More on Reconstructions chapter for more information about rotation hierarchies). This rotation hierarchy can then determine the equivalent absolute rotation of a plate relative to the top of the hierarchy (the anchored plate).

A Reconstruction Tree layer has the following configuration options:

Since this type of layer does not produce visible geometries it does not have the visibility icon to enable/disable visibility.

Instead it has the icon to set/indicate the default Reconstruction Tree layer - see Default Reconstruction Tree below.

The Input channels section has one type of input:

View Total Reconstruction Poles displays a dialog to view a variety of information about the reconstruction poles and the plate hierarchy for that particular Reconstruction Tree layer (at the current reconstruction time). See the Reconstructions chapter for more information on that dialog.

One fundamental difference between Reconstruction Tree layers and other types of layers is you can set a default Reconstruction Tree layer. Only one Reconstruction Tree layer can be the default and you can tell which one is the default because it will be the only layer with the icon visible next to the layer name.

Selecting another Reconstruction Tree layer with no visible icon will make it the new default.

When a feature collection (containing rotation features) is loaded, its associated Reconstruction Tree layer becomes the new default Reconstruction Tree layer. If you want your previous default Reconstruction Tree layer to remain as the default (when subsequent rotation files are loaded) you will need to check the Keep as default tree upon file open check box. This prevents subsequently loaded Reconstruction Tree layers from becoming the default.

The default Reconstruction Tree layer is only applicable if another layer (such as a Reconstructed Geometries layer) requires a Reconstruction Tree input and has not explicitly connected one to its input.

This layer reconstructs features from one or more feature collections using the current reconstruction time. Typically for each input feature geometry there is a corresponding reconstructed geometry (a rotated version of the present-day geometry). This layer is designed to handle different reconstruction methods in the one layer type. Examples of reconstruction methods include rigid plate rotation and half-stage rotation (such as at a Mid-Ocean Ridge).

In order to rotate the present-day geometries of features, a rotation hierarchy is required and this is obtained by connecting a Reconstruction Tree layer.

A Reconstructed Geometries layer has the following configuration options:

The visibility icon determines whether the reconstructed geometries are drawn in the globe and map views.

The Input channels section has two types of input:

The following is an example of an implicit connection to the default Reconstruction Tree layer (because there is no explicit connection)…

…if you then changed which layer was the default Reconstruction Tree layer then the new default would be implicitly connected. This is useful if you have a lot of Reconstructed Geometries layers open and you want to change the Reconstruction Tree layer that they all use without having to reconnect each layer individually. In this case you would just need to change the default Reconstruction Tree layer.

On the other hand if you explicitly connect a Reconstruction Tree layer then the default is ignored (until you explicitly disconnect it).

Note that, in this example, "Add new connection" is disabled (and greyed out) since only one Reconstruction Tree input connection is allowed. You can still have multiple rotation feature collections as input to a Reconstruction Tree layer though.

Set VGP visibility displays a dialog to specify how Virtual Geomagnetic Pole (VGP) features are displayed. This option only applies to VGP features - for other feature types these settings are ignored.

Draw Style Setting displays a dialog to control the colouring of features - see Manage Colouring

The Filled polygons check box can be selected to colour fill features containing polygon geometries. Currently the colour of each filled polygon will be the same as the polygon outline colour (ie, same colour as unfilled polygons).

The following image shows filled polygons for the static polygons in the GPlates sample data.

This layer can display a single raster feature (containing a single raster image or a time-dependent sequence of raster images) in the following configurations:

A Reconstructed Raster layer has the following configuration options:

The visibility icon determines whether the raster is drawn in the globe and map views.

The Input channels section has three types of input:

Opacity controls the transparency of the raster allowing layers drawn underneath a raster layer to become visible through the raster to varying degrees.

Intensity differs from transparency in that it only darkens the raster but does not allow layers underneath to become visible through the raster.

If the raster is non-RGBA (such as a floating-point NetCDF raster) then there are extra options in the Raster options section related to colour palettes.

In the "Raster options" section you can configure the colour palette, for a specific raster band, used to convert each floating-point pixel value to an RGB(A) colour value by selecting a CPT file. Note that this only applies to rasters that are not already in RGB(A) format - see the Data File Types chapter for more information on raster formats. CPT files come in two forms - categorical and regular. Categorical is typically used for non-numerical data (where interpolation of values is undefined). Regular is for numerical, continuously-varying data and is more applicable for rasters. The regular CPT file allows the user to map floating-point pixel values to colours with linear interpolation inbetween.

Selecting "Use Default" will map floating-point pixel values to a small set of pre-defined arbitrary colours. Pixel values two standard deviations away from the mean pixel value will be continously mapped to the small range of colours (with linear interpolation between the colours).

This is the default configuration where no input channels are connected (except the raster feature itself).

The raster is rendered as a non-rotating (or non-reconstructing) georeferenced raster (in this example a global raster). Changing the reconstruction time makes no difference unless the raster feature is a time-dependent raster in which case the image itself will change over time (but will still remain stationary on the globe)…

This configuration does everything the above configuration does (including resolving a time-dependent raster over time) in addition to reconstructing the raster.

The reconstruction is peformed using a set of static polygons. Conceptually the single raster image (or time-resolved raster image for a time-dependent sequence) is cookie cut into multiple polygon-shaped pieces using the present-day location of each static polygon. Then each polygon is reconstructed using its reconstruction plate ID. As each polygon is reconstructed back in time it rotates independently (for polygons with different plate IDs) and transports its cookie-cut piece of raster image with it.

Only polygons whose valid time range (between age of appearance and disappearance) includes the current reconstruction time will be rendered. This is most noticeable near mid-ocean ridges where long thin polygons adjacent the ridge appear/disappear as you go fowards/backwards in time to simulate accretion or crust material at the mid-ocean ridge. This is also the reason why a reconstructed global raster covers the entire globe at present-day but covers a progressively smaller area of the globe as you reconstruct back in time.

This configuration is obtained by connecting the "Reconstructed polygons" input channel to a Reconstructed Geometries layer containing static polygons.

The resulting reconstructed raster…

…note the thin gap along the mid-ocean ridge between South America and Africa. This is an example of a thin ridge-aligned polygon popping out, as you reconstruct backwards in time, because its time of appearance is after the current reconstruction time (34Ma in the example).

This configuration builds on the previous configuration "Configuring a raster that is reconstructed using static polygons" by adding an age-grid raster.

When an age grid is not used the static polygons pop in and out as whole polygons when the reconstruction time changes. Thus the subduction and accretion of oceanic crust is simulated using lots of thin polygons with small differences in age. The age grid takes this even further by providing per-pixel (rather than per-polygon) age comparisons to provide a more continuous transition at plate boundaries. Here the age of the pixel is used instead of the age of the polygon.

This configuration is obtained by connecting the "Reconstructed polygons" input channel to a Reconstructed Geometries layer containing static polygons and connecting the "Age grid raster" input channel to a Reconstructed Raster layer containing an age grid.

The resulting reconstructed raster (with the assistance of an age grid)…

…note the absense of the thin gap along the mid-ocean ridge between South America and Africa. This is due to the per-pixel age comparison (as opposed to the per-polygon age comparison).

This layer generates dynamic plate polygons by closing the plate boundary at each reconstruction time. The plate boundary consists of a sequence of regular features whose geometry is reconstructed and then stitched together to form a closed polygon region for each plate polygon feature. See the Topology Tools chapter for more information of topological features.

A Resolved Topological Closed Plate Boundaries layer has the following configuration options:

The visibility icon determines whether the resolved topological closed plate polygons are drawn in the globe and map views.

Here is an example of turning off the visibility of the Reconstructed Geometries layer so that only the topological polygons are visible.

The Input channels section has two types of input:

Draw Style Setting displays a dialog to control the colouring of features - see Manage Colouring

The Filled polygons check box can be selected to colour fill the topological polygon geometries. Currently the colour of each filled polygon will be the same as the polygon outline colour (ie, same colour as unfilled polygons).

This layer generates dynamic plate polygons in a manner similar to a Resolved Topological Closed Plate Boundaries layer with the addition of deforming the plate region. See the Topology Tools chapter for more information of topological features.

A Resolved Topological Networks layer has the following configuration options:

The Input channels section has two types of input:

The various Show… options under Network & Triangulation options are used to display different aspects of the triangulation generated in the deforming region. These are mostly debugging and visualisation aids.

Draw Style Setting displays a dialog to control the colouring of features - see Manage Colouring

This layer calculates plate velocities at a set of static locations. Here static means non-rotating (the points do not move across the globe as the reconstruction time changes).

The velocities are calculated by determining which topological closed plate polygon contains each static point location. Then the finite rotation corresponding to that plate polygon’s reconstruction plate ID is used to calculate the velocity at the static point location.

This type of layer is automatically created when a feature collection containing features of type gpml:MeshNode is loaded. These features contain multi-point geometry that defines the set of static locations at which to calculate velocity.

An associated Reconstructed Geometries layer is also created to visualise the point locations. The Calculated Velocity Fields layer visualises the velocities as arrows (longer arrows represent larger velocities).

The velocity arrows are not drawn at every static point location. Instead they are spaced apart so that they are minimum distance from each other on the "screen" (on the computer monitor). This so they can be distinguished visually even for very dense meshes.

A Calculated Velocity Fields layer has the following configuration options:

The visibility icon of the Calculated Velocity Fields layer determines whether the velocity arrows are drawn in the globe and map views. And the visibility icon of the Reconstructed Geometries layer determines whether the set of static points (at which velocities are calculated) are drawn.

The Input channels section has three types of input: