MapInfo Pro

MapInfo Monday: Splitting Polygon with Polyline

  • 1.  MapInfo Monday: Splitting Polygon with Polyline

    Posted 30 days ago
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    Happy #MapInfoMonday,

    Today we will dive a bit into some of the tools that you can use in MapInfo Pro when modifying existing spatial objects​.

    In the example, we have a property that we need to split into two polygons. The new boundary is defined to be 125 meters from the existing northern boundary and should follow this 125-meter distance.

    To do so, I'll first draw a line that traces the existing boundary. Then this new line will be offset the 125 meters into the existing property polygon, and finally, I will split the property polygon with the offset polyline.

    Create Line along Existing Boundary

    I start by creating a new help table, that I can draw the polyline into. I base this new table on the Cadastre table to ensure they are using the coordinate system.

    The new table gets added to my map window and is automatically made editable.

    From the Spatial tab, I select the Polyline tool from the Insert dropdown. With the Map window as the active window, I hit the 's' key on my keyboard to enable snap.

    I now place the cursor on top of the first node of the boundary I want to trace. I notice the snap cross appearing and I use the left mouse click to create the initial node of the polyline.

    I then move the cursor to the end of the northern boundary, ensure the snap cross again is visible, hold down the Ctrl key on my keyboard to activate trace, and check that the drawn line is following the desired boundary. I then use a double left mouse click to trace the boundary and end my line.

    In the map below, you can see the traced line shown with a purple override style.

    Offset Polyline into Polygon

    The next step is to offset the polyline 125 meters to the south where the new boundary needs to be.

    I select the drawn polyline and from the Transform dropdown on the Spatial tab, I select the Move Along command. If you can't see this option, make sure you have loaded the MapCAD tool.

    The Move Along tool lets me specify a direction and a distance by drawing a line on the map. I snap to the right end of the drawn polygon and drag a line southwards to the next node on the eastern boundary of the property. Here I release the left mouse button.

    The Move Along dialog now appears prompting me to check the direction and distance calculated from the line drawn. I change the distance to 125 meters and hit the Enter key on my keyboard to exit the dialog.

    The polyline now gets offset by the entered distance and direction.

    Split Polygon with Polyline

    To split the existing polygon with the new polyline, I select the polygon, make the Parcel layer editable, and make the selected polygon the target using the Set Target command on the Spatial tab. I can also use Ctrl + T to set the polygon as the target.

    Then I select the new polyline again as that's the polyline I want to use to split the polygon.

    From the Split dropdown on the Spatial tab, I activate Split Target using Polyline.

    MapInfo now calculates a Cutter object using the selected polyline. The polyline is extended to the minimum bounding rectangle of the target polygon, and from this, the Cutter object is created. MapInfo Pro shows you on the map how the polygon will be split. You can accept this using the Next button or cancel the process.

    In the next step, you can control how to deal with the attributes during the split process. The values specified here will be applied to the two resulting records. Mostly, you would want to keep the values. For numerical values, you can also let MapInfo Pro calculate the values using the Area Proportion method. That could be useful for example for values that are linked to the size of the polygon, like area or maybe even population.

    And here is the final result where the polygon has been split using the drawn polyline. I have only selected the northern part of the polygon to show that it has been divided from the property.

    If I don't need the HelperLines table anymore, I can just delete it.

    I hope that you found this little tip useful. Are you doing a lot of data editing with MapInfo Pro? If so, you might also find the document useful: Data Editing in MapInfo Pro v2019.

    Peter Horsbøll Møller
    Principal Presales Consultant | Distinguished Engineer
    Precisely | Trust in Data