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Surface data files consists of an ordered array of interpolated values at regularly spaced intervals that represent the spatial distribution of an attribute, such as elevation, elevation models, temperature, concentration. The Map Viewer in Hydro GeoAnalyst supports SURFER Grid Files (*.grd) and ESRI ASCII Raster Files (*.asc).


Adding an existing Surface (Grid) to the Map

To add a grid to the Map Viewer, select the Add layer [] button, select the Add Surface option and navigate to a supported layer file.


Creating a Surface (Grid)

To create a grid, select a point layer in the Map Layers tree and select the create grid [] button from the Map Layers toolbar. This will open the Create Grid Window:


The following settings will be provided


Grid file: The location where the grid file will be stored. The default location is in the .\Map\Grids subfolder in the current project.


Interpolation Options:

oInterpolate with point X values: this will use the elevations embedded in the shapefile. Note: this value is not defined (NaN) for project data driven layers (including Spatial Queries, Station Groups, and Sample Sets).

oInterpolate with attribute: this will interpolate values from the specified attribute field. Note: you can add attributes to a Station Group or Sample set using the Map Data Settings for that layer.


Restrict/extrapolate to polygon extent: allows you to define the interpolation domain/grid extents based on the extents of a specified polygon in Map Tree. If checked, the interpolation domain (Xmax, Ymax, Xmin, Ymin) will be updated to the rectangular extents of the specified polygon layer.


Interpolation Settings: a collection of input settings that define how the input values will be interpolated to the grid. Please see the Interpolation Methods and Settings section below for more details


Clicking the Create button will generate a grid file based on the provided settings and add the new grid to the Map Layers Tree.


Interpolation Methods and Settings

Select the interpolation method to use for generating the surface. Choose from the following interpolation methods:

Inverse Distance


Natural Neighbor


A complete description of the interpolation methods, which are part of a large and complex branch of statistical sciences, is beyond the scope of this help documentation and a working knowledge of this topic by the user is assumed. The methods implemented in Hydro GeoAnalyst are based on the public domain Geostatistical Software Library (GSLIB), written by Deutsch and Journel (1997) and distributed by Stanford University. A brief summary is provided below; however, interested users or users who are unfamiliar with these methods should consult references and resources.


Inverse Distance

The Inverse Distance Weighting method is very a fast and efficient weighted average interpolation method. The weighting factor applied to the data depends on the distance of each point from the grid cell, and is inversely proportional to the distance squared. Consequently, the greater the distance the data point is from the grid node, the smaller the influence it has on the calculated value at the grid node.  The Inverse Distance method for interpolation may generate patterns similar to the “bull’s-eye” surrounding points of observations. Selecting a larger number of nearest neighboring data points may smooth this effect, but if the bull’s eye pattern is undesirable, then other methods for interpolation, like Natural Neighbor and Kriging, are recommended.



Kriging is a group of geostatistical techniques that are based on Gaussian process regression of spatial input data. Anisotropy and underlying trends suggested in the raw data or inferred by expert knowledge of the site and associated geologic processes can be incorporated in an efficient manner through Kriging methods. Hydro GeoAnalyst incorporates kt3d, a public-domain program in the Geostatistical Software Library (GSLIB), written by Deutsch and Journel (1997) and distributed by Stanford University. All kriging methods provide minimum error variance estimates of values at unsampled locations and rely on the concept of auto-correlation - that is, the values of samples nearer to one another are more related to each other than samples that are farther apart. There are several multiple ways in which the relationship between proximate samples can be estimated/modeled based on the selected method of kriging. Hydro GeoAnalyst includes the following kriging methods:

Stationary Kriging: stationary kriging (also known as simple kriging) is a basic generalized linear regression of local departures from the known (or specified) global mean value

Ordinary Kriging (default): Ordinary kriging is a special case of simple kriging that automatically estimates the global mean value such that the kriging weights add up to 0.

Nonstationary: non-stationary kriging (also known as linear drift kriging, kriging with a trend model, or regression kriging) is a more generalized form of simple kriging that assumes that there is an underlying (linear) trend inherent in the data

External Drift: external drift kriging (also known as universal kriging) is a special form of non-stationary kriging where local and regional trends of the values are unknown, but the variance of the variable is assumed to be constant.


Kriging methods rely on the use of a semi-variogram which describes the degree of spatial dependence of values between points on in the field.  If the semi-variogram components have already been modeled or estimated by the user, they can be incorporated into the program by choosing the appropriate set of parameters in the advanced settings. The semi-variograms available include Spherical, Exponential, Gaussian, Power, and the Hoe effect models. If the variogram information is not available, the default linear variogram with no nugget effect should be used. This option is a special case of the Power model with the exponent equal to 1.  When using the Kriging interpolation method, you may need to adjust the min and max radii of the variogram in order to get an ideal interpolation; these parameter values will depend on your data set.  Default values are 100 and 100 along the major and semi-major axes. You will likely need to alter these values based on your input dataset and on the characteristics of the model domain.


Natural Neighbors

The Natural Neighbor method (Sibson, 1981 and Watson, 1994) is based on a Voronoi tessellation of the Euclidean plane about a specified number of the nearest input data points. Voronoi diagrams are often called Thiessen polygons, which are method commonly used in meteorology and related geosciences. The grid node for interpolation is considered a new point, or target, to the existing data set. With the addition of this point, the Voronoi polygons based on the existing points are modified to include the new point. The reduction in area of the existing polygons to include the new point is called the “borrowed area”. The interpolation algorithm calculates the interpolated value as the weighted average of the neighboring observations where the weights are proportional to the borrowed areas. The Natural Neighbor method is valid only within the convex hull formed by the data points, and values extrapolated outside of the hull should be used with caution.

The Natural Neighbor interpolation scheme may be visualized as a taut rubber sheet stretched to satisfy all the data points. The interpolated value at any location is a linear combination of all Natural Neighbors of that location, and the resulting surface is continuous with a slope that is also continuous. Combining the gradients or slopes with the linear interpolation provides results that are more smooth, and may anticipate peaks and valleys between data. Singularities and other undesirable effects may be reduced by incorporating the gradient factor.

The gradient influence on the results can be manipulated by two tautness parameters that you can enter. These parameters allow the interpolated surface to vary from purely linear interpolation to one which is well rounded and has a gradient factor. In all cases the slope discontinuities are removed and the resulting surface has slope continuity everywhere.


General Settings

The following general settings are used for interpolation:

Min Value: minimum output value in the grid (by default this is the minimum input value)

Max Value: maximum output value in the grid (by default this is the maximum input value)

Interpolate Log Values:

oIf set to yes, then the logarithm of the values will be interpolated and converted back to the anti-logarithm values once interpolated. This setting is useful if the values are log-normally distributed, which can the case for many naturally occurring systems.

oIf set to no, then the untransformed data will be interpolated


Note that some interpolation methods, particularly kriging, can result in values outside the range of measured values and in output values collacated with input locations that do not match the input values. In such cases, it may be desirable to truncate the output to a specified minimum and maximum value.


Grid Settings

The following settings are used to define the grid over which the surface will be defined:

X intervals: the number of columns in the grid along the x-direction

Y intervals: the number of rows in the grid along the y-direction


Interpolation Domain

The following settings are used to define the extents of the grid over which the surface will be defined:

X Max: the maximum X-coordinate (at the eastern edge) of the grid

Y Max: the maximum Y-coordinate (at the northern edge) of the grid

X Max: the minimum X-coordinate (at the western edge) of the grid

Y Max: the minimum Y-coordinate (at the southern edge) of the grid


Advanced Settings

The following settings are available depending on the selected interpolation method:

Advanced Settings for

Inverse Distance


Advanced Settings for



Advanced Settings for

Natural Neighbors



Grid Settings (Surface Colors)

Surfaces added to the map layers will be rendered as using a color ramp that maps values to colors to values that adds or imbues meaning and appeal to visualizations of the surface grid. By default, grids are rendered with a black to white color ramp. To change the colors of the ramp, select the surface in the Map Layers tree and click the Grid Settings button in the Map Layers toolbar. This will open the Surface Colors window:



The Surface Colors window contains the following settings:


Color Ramp Definition Table

The color ramp definition table consists of two or more rows of color stops. Each color stop in the table consists of a specified color that corresponds to both a position along the color ramp (from 0 to 1) and a corresponding value of the surface. Colors in between the color stops are interpolated in in the red, green, blue (RGB) color model. A preview of the resulting color map is shown in the preview at the top of the window.  Clicking on a row in the table makes that row the active color stop as denoted by the arrow symbol on the left and blue highlighting []. Settings for the active color stop are displayed and editable in the right side of the window and are described in the "Color stop settings" section below.

The color ramp definition table includes a toolbar with the following controls:


Add color stop: adds a color stop to the color ramp at the mid point between the active color stop and the row below

Delete color stop: removes the active color stop from the table

Dropdown: allows you to set the number of color stops (only works with the auto-ramp button)

Auto-ramp: recolors all of the stops based linear interpolation of the RGB values of the first and last stop

Smooth All: sets all color stops to continuous color stops

Classify All: sets all color stops to hard color stops so that the first stop of a given color is the last stop of the previous color so that surface values between stops are all one color

Save: saves the color ramp definition table to the color ramp library in the color ramp editor

Load: opens the color ramp editor

Opacity: changes the relative opacity of the surface (100 is completely opaque, 0 is completely transparent)

Logarithm: changes the scale of the surface values relative to the color ramp space:

oIf unchecked: the surface values are scaled linearly relative to the color ramp (default)

oIf checked: the logarithm of the surface values are scaled linearly relative to the color ramp


Color stop settings

Each color stop has the following settings:

Position: the position of the color stop along the color ramp between 0 and 1

Value: the corresponding value of the surface

Hard Stop Checkbox: a setting for whether the color stop is a hard or smooth

oIf checked: the color stop is a hard stop with a different color above and below the color stop

oIf unchecked: the color stop is a smooth stop such that the color ramp transitions through the color stop smoothly from above and below


Update: the update button commits any changes you've made to the settings above to the active color stop in the color ramp definition table


Color Ramp Editor

The color ramp editor contains a list of pre-configured color ramps that can be used to render surfaces. You can delete [], rename [], export [], or import [] a color ramp from a .clr file. Any changes made to the list of color ramps can be saved [].  Clicking OK will load the active color ramp in the Surface Colors window, while clicking Cancel closes the color ramp editor without changing the Surface Colors window. A preview of pre-configured color ramps available with HGA is shown below:



Contour Settings

Surfaces added to the map can be contoured - that is Hydro GeoAnalyst can use a surface to generate a set of polylines representing lines of constant value in the surface. Contours can be added as either major or minor contours. There is no real difference between major and minor contours except that the distinction allows you to assign separate settings. To generate major/minor contours for a given surface, select the surface in the Map Layers tree and click the Create Contours button in the Map Layers toolbar. This will open the Create contours window:



The create contours window contains two main sets of controls:


Contour Settings

The left side of the Create contours window contains settings for a single contour or range of contours that you can add to the map as either major or minor.

The first group of settings allows to define which contour(s) will be added to the map:

Add single value: a single specified value that can be added to the map

Add range: a range of values based on the following settings:

oFrom: the lower value in the range

oTo: the upper value in the range

oEvery: the increment between values


The second group of controls allows you to add the contours to the map:

Do no add duplicates: if checked (default), then any values already added as either major or minor contours will not be added to the map. If unchecked, then duplicate values will be added to the map

Contour type: allows you to select if the contours to be added (based on the settings above) will be major or minor contours

Add Contours: clicking this button adds contours to the map based on all of the settings above


Added Major/Minor Contours

The right side of the Create contours window contains a list of the major contour values and a list of the minor contour values with the following distinct settings/controls for each:

Checkbox: the checkbox above the list toggles whether the major/minor contour(s) are shown on the map

Delete: deletes the selected contour(s) from the list

Clear: deletes all contours from the list

Labels: label/format settings for the contours (major contours only)

Line Settings:

oColor: click on the color to edit the color of the contour line(s)

oThickness: the number value defines the thickness of the contour line(s)

oType: the dropdown defines the style of the contour line(s) - solid, dash, dot, dash-dot, dash-dot-dot



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