Adjusting GIS element's position, to align precisely with the coordinates of another layer
A SGIS Case Study on “Snap By Layer” Function
Published on: 10.10.2023
Changing the geometry of elements involves adjusting their coordinates.
GIS data encompasses elements with point, polyline, or polygon shapes, along with associated attribute data. To determine the coordinates for creating the geometry of GIS elements, you can consider various sources, such as:
- Precise land surveying measurements,
- LIDAR measurements,
- Utilizing CAD software for elements that can’t be directly measured,
- Employing manual GPS devices or obtaining GPS coordinates from mobile phones,
- Converting raster data to vector data from cadastral plans,
- Converting raster data to vector data from topographic maps,
- Converting raster data to vector data from satellite images,
- Extracting coordinates from various web services.
It’s important to note that vectorization is conducted based on georeferenced backgrounds.
Different sources for obtaining element coordinates can lead to varying levels of precision in the coordinates themselves.
When creating GIS data, there are two primary approaches:
- Utilizing ‘precise’ coordinates for geometry initially.
- Starting with approximate coordinates in GIS data, which are later refined when more accurate coordinates become available.
In essence, the need to adjust the geometry of elements in a layer typically arises from either:
- Upgrading less precise coordinates to higher precision.
- Replacing existing coordinates with officially accepted ones.
In this text, we’ll outline a method for modifying element geometries using SGIS Desktop software.
In the context of this text, ‘geometry change’ refers to the adjustment of the position of one or more points within an element’s geometry by modifying their coordinates. It doesn’t involve changing the number of points forming the element (polyline or polygon) by adding or removing vertices.
When we mention a ‘coordinate change’ in this text, it means modifying either one or both coordinates of a single point. These coordinates can be in projection or on an ellipsoid. The process of altering an element’s geometry is a part of GIS data maintenance.
SGIS Desktop offers two primary methods for geometry modification:
- You can directly adjust the geometry of a selected element by modifying its XY or Longitude and Latitude coordinates using the ‘Modify Element Coordinates’ action.
- Alternatively, you can modify the geometry of multiple elements using the ‘Snap By Layer’ action.
Changing the Geometry of a Selected Element
This method of modification is a general approach and implies that the user possesses ‘accurate’ coordinates that need to replace the existing ones for one or more points within an element. It provides flexibility for users to make precise adjustments to the element’s geometry.
To modify an element’s geometry, follow these steps:
- Select the element you want to alter.
- Choose Edit > Element Coordinates > Modify Element from the menu, or click the Modify Element Coordinates icon in the ELEMENT DATA toolbar.
- A window will open, presenting a text editor with the vertex coordinates of the selected element. These coordinates are listed in the format X Y or Longitude Latitude, separated by a single space.
- You can update the coordinates by either loading them from a text file, copying them from the clipboard, or making direct edits in the text editor.
- Execute the geometry change by using the action Do > Change Coordinates Of Existing Element. The modification will be instantly reflected in the graphical representation.
It’s important to note that this method allows not only coordinate adjustments but also comprehensive changes to the vertex points that define the selected graphical element.
This is especially significant when dealing with polyline or polygon-type elements. In the case of point-type elements, only a single coordinate can be modified.
Geometry Adjustment using Snap By Layer
The Snap By Layer action initiates a new interface. Within it, you choose both the layer you intend to modify and the layer that acts as the criterion for changing coordinates. When selecting the layer for modification, you’ll encounter a section called the Snapping Circle. Here, you specify a tolerance in the form of a circle, with the radius defined by the user. This tolerance is measured in meters.
Before committing to the Snap By Layer action, it’s advisable to use the Calculate Distance feature. This helps identify the largest coordinate deviations between points in the layer being modified and the reference layer, which represents the precise positions of these points. Based on this analysis, you can then select an appropriate tolerance for the Snap By Layer action.
In our example, we typically observe differences of approximately 10 cm, with the largest variance reaching 20 cm. This scenario involves an optical cable layer in telecommunications, where we need to reposition the coordinates of intersection points to align with officially accepted standards. The reference layer contains 3369 points that have received official approval, while the layer undergoing modification consists of 32 polylines.
Observing the scenario, we notice that there are points not only at the locations where the optical cable is routed. The reference layer can encompass not only point features but also polylines or polygons. We proceed by opening the form using the Snap By Layer action.
After defining the layer to be moved, the reference layer for the relocation, and setting the tolerance, simply click the ‘Move Layer Points’ button to initiate the relocation process. The status bar will display the count of points moved within the specified tolerance. You’ll also notice a convenient button for saving a LOG file of the moved points, providing you with the option to store this file on your disk. The outcome of the relocation process can be observed in the graphical representation.
The same process is applied to any other layer requiring coordinate adjustments. When relocating parcels, there’s no need to be concerned about gaps or overlaps.
This action significantly reduces the time and effort that employees would otherwise spend manually snapping using CAD methods.
Note: You can load the saved LOG file as a point-type layer to visualize the relocated points, offering a graphical overview of the point relocation process.