Example: Trace an Area in a Map over an Image Background

In this example we use a map with a drawing layer above an image layer.  We will create an area object in the drawing by tracing over the outlines of something seen in the image layer below.


To set up this example we first create a new data source using an image server, the Google Maps Satellite Image server.   We then create a map and drag and drop the image server image into the map.


Next, we create a new drawing, called TDrawing by default,  using the default  pseudo Mercator projection and then drag and drop that into the map.


To see a step by step tutorial on creating an image server, adding it to a map as a layer and then creating a new drawing and adding it to the map as a layer, see the Example: An Imageserver Tutorial topic.



We zoom into France and then into the center of Paris.   In the center of the view above we see the oval road running around the Place de la Concorde with the Tuileries gardens to the right.




We zoom further into the view to see the octagonal, shallow pond where hobbyists float model sailboats in the summer and everyone relaxes in lounge chairs around the edges.  We will create an octagonal area object in the drawing, "traced" over the pond.




As described in the Editing Drawings topic, we choose Create Area for the mode button in the main toolbar.




We click on one corner of the octagonal pond.




We then click on the next corner.    As we click and move the mouse cursor onwards we see a small box has appeared to mark the coordinate where we clicked.   A thin blue line will "rubber band" to connect the last place we clicked to the current position of the mouse cursor.




As we continue clicking and moving the mouse cursor we can see that the rubber-banding blue line shows us what the area object would look like if we were to click at the current mouse cursor position.




We click all the way around the octagon.




We click the last corner of the octagon...




...and then we Right-click anywhere to indicate we are finished drawing the area.




In the resulting context menu we choose Save Changes.  



The area object is created in the drawing.



When finished drawing areas, do not forget to switch back to Default mode, which provides navigation mode by default.




There is no harm done if we forget, since immediately any cursor clicks in the active window will begin to create an area.  We can press Esc at any time to abandon the edits, and no area will be created if we do not choose Save Changes or otherwise confirm the edit.



Zoom in for Greater Accuracy - We created the area object in a casual way, clicking our way around the octagonal without bothering to zoom it or to pay particularly careful attention to where exactly we clicked.   If we were digitizing images for professional work we would be more careful to click with greater accuracy.


To click with greater accuracy we can zoom in using the mouse wheel or by clicking the Zoom In or Zoom Out buttons in the main menu.   




For example, in the view above we have used the mouse wheel to zoom in to one of the corners of the octagon to more precisely choose where we will click to put a coordinate for the area.


We can also click and drag to pan the view.   As long as we click and drag as opposed to just clicking without doing a drag, the system will read our mouse gesture as a command to pan and not as a command to put a coordinate box at that spot.


Pseudo Mercator Coordinate System - Since the drawing in this example contains but one object, a tiny amount of data, we do not need to worry about the speed of re-projecting the drawing on the fly to match the coordinate system of the map.  We used the default Pseudo Mercator coordinate system, which happens to match the coordinate system used in the map as a result of the Bing imageserver being dropped into the map.  That is one reason that Pseudo Mercator is the default within Manifold, since many people will use web server imagery in that projection as a basemap.   If desired, we could have used a different projection for the drawing.   Usually if a drawing has many objects in it we would want to match the coordinate system of the map exactly, but Manifold is so fast that is not a big deal in smaller or mid-sized drawings.

See Also

Example: An Imageserver Tutorial - An extensive tutorial showing step by step how to add new data sources that are imageservers, how to show them as layers in a map, how to create a new drawing that matches the projection of the map and how to trace over what is seen in an imageserver layer to create an area object in the drawing.


New Object Dialog


Example: Draw Lines, Areas and Points - Simple example of using basic mouse moves to add points, lines and areas to a drawing.


Example: Create a Line using the Record Panel - Step by step creation and modification of a line in a drawing using the Contents - Record panel's Coordinates tab.


Example: Create a Line using Curved Segments - Creating a line made up from curvilinear geometry using the New Object Dialog.


Example: Create an Area with a Hole - Using the New Object Dialog, create an area in a drawing where the area includes one or more holes.  This is similar to how we create areas that have islands as part of the area.   


Example: Create an Area with Holes and Islands - Using the New Object Dialog, create an area in a drawing where the area includes holes and also islands.


Example: Create a Multipoint - How to create multipoints using the New Object Dialog.  This topic provides two examples:  First we create a multipoint and then next we create a multipoint having two branches.  The purpose of this topic is to help teach the implementation of geometry in Manifold and other typical spatial packages using a somewhat unusual and rarely met object type, the multipoint, which combines what appear to be many separate points into a single multipoint object.


Example: Style Applied to an Image Server Image - Because the Edit - Style dialog simply changes the way an image is displayed and not the data, it can operate on read-only data served by various web servers such as WMS REST servers.    In this example we look at every detail of creating a data source using an image server and then manipulating the appearance of the display with Style.  We will connect to a WMS server that provides LiDAR data in various forms, including as terrain elevation.