Convert Coordinates
When vector objects or data points in a base layer are not spatially referenced correctly, the Map Tools | Layer Tools | Convert Coordinates feature can be used to reference the objects correctly.
This feature is primarily used to convert a base layer (either vector layer or data points layer) from an unknown coordinate system to either a known or an unknown system. For example, the feature can be used to convert from a local system to a UTM system. Or, the feature can be used to convert from one local system to another local system.
If you need to spatially reference an image being used as a base layer, the Map Tools | Layer Tools | Georeference Image can be used, instead.
Convert Coordinates Dialog
The Convert Coordinates dialog is opened by clicking Map Tools | Layer Tools | Convert Coordinates or by right-clicking the base layer in the Contents window and clicking Convert Coordinates....
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In the Convert Coordinates window, either a Math Operation or Geo Conversion can be performed. |
The Convert Coordinates window contains two primary sections for the two kinds of coordinate conversions this feature can do, Math Operation and Georeference.
Math Operation
The Math Operation option applies a mathematical operation to the X and/or Y coordinates. This option can be used to adjust for offsets or errors in the data, like if the coordinates were off 50 meters in one direction. This option can also be used to convert units. For example, if units were measured in meters and they should be converted to feet, select the Multiply by operation and enter 3.28084 in the value field. The following math operations are available:
| Add | Adds the value in the value field to the target coordinate(s) |
| Subtract | Subtracts the value in the value field from the target coordinate(s) |
| Multiply by | Multiplies the target coordinate(s) by the value in the value field |
| Divide by | Divides the target coordinate(s) by the value in the value field |
| Mirror | Mirrors the data across the midpoint of the target coordinate(s). When Mirror is selected, the value field is disabled. |
Georeference
The Georeference option can be used to re-calibrate coordinate information for the layer based on at least three points on the map with known real-world coordinates and a chosen Warp Method. For example, if data was collected in a local coordinate system, it could be converted into a known geographic coordinate system.
The feature can be used by toggling the Georeference option, selecting a Warp Method, and clicking Add to add control points to the Base map. Once points have been added, target values for the X and Y coordinates can be entered in the Target X and Target Y fields. Once finished, click OK to complete the conversion.
Source Coordinate System
The Source Coordinate System is the coordinate system assigned to the layer being edited. This cannot be edited in the Convert Coordinates window. The linear units for the source data can be defined using the drop-down box. If the units for the source data are lat/long, the field should be left as <unknown>.
Target Coordinate System
The Target Coordinate System is the coordinate system the data will be converted to. The Target Coordinate System can be adjusted by clicking the Change... button. This opens the Assign Coordinate System dialog.
Control Points
The Control Points section of the window allows the Warp Method to be adjusted, control points to be added, removed, and edited, and georeferencing files to be loaded or saved.
Control points can be added manually or by loading a georeferencing file. To add points manually, click the Add button, bring your cursor over the desired area on the Base map, and left-click to add a point to that area. Points can also be added manually by clicking the Add button and manually entering a value for the Source X and Source Y fields. To add points using a georeferencing file, click the Load... button. In the Open dialog, navigate to your georeferencing file, select it, and click Open. The points are loaded automatically.
Control points can be moved by selecting the row for the desired point in the control points table and left-clicking on a new location of the map. Control points can also be moved by adjusting the Source X and Source Y values. Once the values are adjusted in the control points table, the control point marker on the Base map will move to the new location.
Control points can be deleted by clicking the icon in the Remove column of the control points table.
To save control points for future use, click the Save... button. In the Save As dialog, name the file, select a file path of your choosing, and click Save. The georeferencing file can be loaded for future conversions.
Control Points Table
The control point table contains information about the source and target coordinates for the control points. At least three control points are required to warp the image. Most warp methods require at least four control points, but there is no limit to the number of control points you can add. At least seven columns are displayed in the control point table. Each row represents a single control point. Any missing values or values that result in errors are highlighted in yellow in the control point table.
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Enter the control point information in the control point table. |
- The Point ID column contains the ID number for the control point. The ID helps you associate rows with points in the plot window.
- The Source X and Source Y columns contain the source coordinate of the control points in the base layer.
- The Target X and Target Y columns contain the desired coordinates of the control points. The Target X and Target Y columns are displayed when the base layer is assigned an unreferenced local coordinate system. Type the coordinate values for the control point in the Target X and Target Y fields.
- The Point Error column displays the RMS error value for each calibration point between the source image and referenced image.
- The Remove column hosts a button that can be clicked to remove a row from the table.
Click a cell to select it. Begin typing to replace the selected cell's contents. Double-click on a cell to enter cell edit mode and edit the current cell contents. When you are finished replacing or editing the cell's contents, press enter or click on another cell.
Warp Method
The warp method determines how the image is warped to the control points. Available options are: Affine Polynomial, First Order Polynomial, Second Order Polynomial, Third Order Polynomial, Thin Plate Spline, Natural Cubic Spline, Marcov Spline, Exponential Spline, Rational Quadratic Spline, and Inverse Distance Squared.
Click the Warp Method drop-down and click the desired method from the list to change the warp method. If too few points exist for a warp method, more control points must be added before the conversion can be completed. The number of required control points is displayed before the warp method name. Affine Polynomial requires at least three control points. Second Order Polynomial requires at least six control points. Third Order Polynomial requires at least ten control points. All other methods require at least four control points. Refer to each specific warp method topic for the transformation information and an image demonstrating the warp.
Applying Changes to the Map
When the steps for the desired coordinate conversion are completed, the conversion can be applied to the Map by clicking OK. If the coordinate conversion has changed the limits of the Map, you will need to either allow Surfer to update the limits of the map automatically or manually adjust the limits to new, appropriate values.
Undoing Changes to the Map
If you wish to revert the changes to the map after using the command, the changes can be undone with the plot window Undo command. After clicking the Undo command in the plot window, the map will revert back to its previous state.
Note, using the Reload Layer Data feature on a layer whose coordinates have been converted using this feature will return the layer to its original, unconverted state. This is because the layer source still references the original file. This file path can be updated by saving a copy of the new layer, either using File | Export or the Save File command for the layer, and then setting the Input path property on the General page for the base (vector) layer to the new file.