Treatment of Vector Data in SPRING

This page presents some definitions about the vector structure and the procedures for the edition of maps of the type thematic, cadastral, networks,  and DTM  through digitalization.

The edition of vector data in SPRING is made on maps of the type thematic, cadastral, networks,  and digital terrain model (DTM).

The geographic objects have intrinsic properties like location (position of an object in geographic coordinates), dimensions (allows the description of the object by entities like points, lines,  or areas), continuity, size,  neighborhood, form, and scale that must be known in order to be represented.

The maps in SPRING can have their representation in the vector and raster formats, depending on the category of the data (see about the conversion of representations). However, in this page we will only deal with the edition of vector data.

For a thematic, cadastral, network, or numeric map  we use elements  like points, lines, and areas (or polygons), to define the thematic classes, geographic objects and numeric samples (contour lines and elevation points)

See also:

Editing Maps in SPRING.
Raster Edition.


Elements of vector structure

Vector data edition in SPRING is performed on thematic, cadastral, network, and DTM (digital terrain model) maps. The vector representation of these maps is the most precise way of representing a geographic object.

In terms of computational implementation the storage and retrieval of this format is more complex than on the case of the raster or bitmap representation. In SPRING it has been implemented a structure of "v-r-trees" to facilitate the access to the data.

Point

    A point is defined as any geographic entity that can be located by a pair of coordinates (x,y).

    Points are used to represent the location of a geographic phenomenon in a place, or to represent some feature in a map that is too small to be shown as an area or line. Examples: location of a city, an airport runway, the summit of a mountain or an elevation point (when it has an attribute Z, besides  the X, Y coordinates, that could be the elevation or some other parameter). The figure below shows a point P as defined by a cartesian system.

Line

    A line is an entity defined by at least two pairs of (x,y) coordinates, that is, two points.

    Lines are used to represent map features that are too thin to be shown as an area or that theoretically have no width. Examples: a river, a road, the coast line of a continent, a contour line or an administrative limit.


    When a line has some attribute Z, besides the  (x,y) coordinates of the points that constitute it, it is called an Contour line In an Contour line every one of its points has the same value of Z. An Contour line is only defined in ILs of numeric category.

Areas

    Areas are defined as a series of (x,y) coordinates forming line segments that close in a polygon.

    Frequently in geographic information systems, areas are represented by polygons. Examples: the geographic extension of a city, a lake, a deforested area.

 

See also:

How to obtain conceptual information and tips

Vector Edition


Vector Representation in SPRING

In SPRING the three elements above are translated into geographic features that are represented by: nodes, points, arcs, contour lines, islands, polygonal lines, and polygons.

Arc

    An arc is a set of points interconnected by line segments that start and end at  node.


    Examples of arcs.

    Arcs are used to model the frontiers of the polygons. This way they are used to delimit objects that define areas.

    A node is a special type of point whose objective is to define the intersection point of two or more arcs. The figure below illustrates a polygon formed by arcs and nodes. Two adjacent polygons can share one same arc, provided the intersection of the lines be delimited by the presence of a node.

    In the following example there are two polygons, the first formed by arcs 1 and 2, and the second by arcs 2, and 3, that could represent rice and soybean areas.


Points

    Points are entities used to represent features that are too small (2D points) to be represented by a polygon or to represent a numeric sample (3D point). A 2D point is normally associated to a symbol in thematic, cadastral, and network maps defined in the database, besides its non-spatial attributes. For example: a church or a lighting pole that cannot be represented in the present scale.


Islands

    Islands are a special type of polygon delimited by one arc only. Only one node defines the initial and the end point of the polygon, since there are no adjacent polygons. The figure below shows an arc that starts and ends in the same node, thus defining an island.


Polygonal Line

    Polygonal Line or open polygon is formed by a set of points interconnected by line segments that begin and end at a node.The difference between a polygonal line and an arc resides in the fact that a polygonal line never defines an area (polygon). It is used to model linear features like, for instance, lines that represent geological fractures, rivers, roads, and other geographic elements that could be observed as linear features in the adopted scale.


    The polygonal line is used when the intersection point of the lines should not be modeled and so there is the need of introducing a node.

Contour Line

    A contour line can be seen as a polygonal line where one only value of Z is given. The figure below shows two contour lines with different heights. More details about contour lines and their usage can be found in Numeric Modeling.


     

See also:

How to obtain conceptual information and tips.

Vector Editing


Topology

Points, lines, and polygons are vector representations normally used to describe geographic objects in maps. The spatial relationship among these entities, like for example, proximity and neighborhood, is obtained by the interpreter from the analysis and observation of the maps.

However, once the objects in the map have been digitized and are represented by points, lines, and polygons in the computer, such spatial relationship should be explicitly defined so that the spatial data analysis operations can be performed.

In digital maps the spatial relationships are described by the topology defined as the mathematical subject that studies the geometrical properties that do not vary with a deformation. Shapes and object coordinates are less important than the elements of the topological model like: connectedness, compactness, continuity, etc...

A simplified form of an example of topological structure that is generated for a thematic map.

This way, defining topology means making clear the spatial relationships among the objects by means of a mathematical process.

Defining the topology of data of the thematic or cadastral model in SPPRING results in the creation of polygons, that is, the system will store the information about the lines, nodes and identifiers that compose each polygon, as well as about the lines that are shared by different polygons (including the neighborhood and circunscrividade (?) among them).

In SPRING the topology, as related to nodes and the neighborhood of arcs, can be automatically defined during digitization. When digitizing a line, a node will be automatically inserted when intercepting other line or at its ends. However, the creation of the polygons must be made so that the whole topology can be defined for the Infolayer.

Once the topology is defined, each polygon can then be associated to a thematic class, or to an object of the cadastral map, or yet, to a network segment, provided they are defined in the Database.

 

See also:

The Conceptual Scheme of SPRING

Vector Editing


Vector Editing

In the process of vector editing in SPRING, specially in cadastral, thematic, and network  maps, the user has to go through the steps of Digitizing, Adjustments, and Polygonalization. For the edition of a numeric IL one only needs the Digitizing and some eventual adjustments.

Digitizing

Digitizing is a process that allows the conversion of spatial data from analog to digital media. Digitally these data are structured in such a way as to allow the execution of operations that are typical of spatial analysis.

The user may digitize the lines by introducing them point after point, in the Pass mode, or simply by following the feature with the mouse continually activated, in the Continuous mode. In this mode you can also define the frequency that the points will be acquired to compose the lines via the Digitizing Step. This parameter corresponds to the interval between the points of the digitized line. This parameter is given in millimeters in the scale of the Infolayer being edited. Check the scale of the IL in Edit - Infolayer in the main menu. Remember that the cartographic precision of maps is in the order of 0.3 mm times the map scale. So a digitizing step that is less than this value will be out of the precision limit of the map itself.

In SPRING the digitization of data can be performed with the definition of Automatic or Manual Topology. In automatic topology each time an arc intercepts another a node will be automatically defined, without the need to have the operator indicating it. This mode is ideal for the digitization of polygons, being valid only for the line being digitized.

In manual topology the introduction of nodes or line breaks should be performed by the operator. This mode is indicated, for example, for the digitization of geologic fracture lines, where one line should remain integral even of others cross it.

The digitization an be performed via different peripherals like the digitizing table, (the most common), image scanning devices or a video monitor (screen).

SPRING allows the digitization of data by the table or by the video monitor as described below.

  • Digitization with the table

The digitizing table is basically constituted of two parts:

- a plane surface, electronically sensitive, where the map or graphic to be digitized is placed;

- a mouse, magnetic device , that sends the (x,y) coordinates of a point on the table to the computer.


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The digitizing table mouse has the function of acquiring the (x,y) coordinates that will be related to the geographic coordinates. This is done through the buttons that play specific roles for each desired objective.

By activating the data acquisition from the digitizing table mouse, the compute mouse is deactivated.

In the manual digitization of data, the buttons of the table mouse have pre-established functions.

  • Button 1 has the function of point acquisition, that is, it can edit points and lines. Corresponds to the select button of the computer mouse.
  • Button 2 sends to the system the information of end of line. Corresponds to the adjust button of the computer mouse.
  • Button 3 has no special function, and so is not used.
  • Button 4 has the function of finalizing the manual digitization of the data via table. When pressed, the table mouse becomes inactive, and the controlling functions get back to the computer mouse.

Before digitizing via the table, we must perform its calibration.

Firmly fix the map over the table, in such a way as to allow no clearances. You must verify that the map lies within the sensitive limits of the table, as shown in the figure above.

 

  • Digitizing with the screen

The user can digitize the lines and points of the map on the screen itself, using the computer mouse for the definition of the geographic objects, according to the description of the mouse buttons:

  • The select button or left button of the mouse makes the line editing; point addition, point movement, depending on the operation selected in the edit window.
  • The menu button or right button defines the end of a line or arc (by inserting a second node). Can be used to show all the points and nodes of a line, independently of the editing function that is selected.

Errors associated to vector digitization

Following we present some errors for the guidance of the SPRING users during digitization.

User has digitized an insufficient number of points: the representation of curved formats depends on the number of vertices used. Consequently, the errors related to the digitization of straight lines is much smaller than the ones obtained by the digitization of complex curves. Example:

The definition of a suitable value for the Digitizing Step can minimize this error. We warn the user, though, to the fact that too small steps will generate lines with too many points.

Since some errors can be avoided and others provoked by the choice of the topology type (manual or automatic), the errors will presented for each topology type.

Manual Topology

A- The user did not define a node - in a polygon, every line that intercepts another line should have a node associated to the intersection point, for example:

In this case, we must insert a point in the line that was intercepted and transform it into a node (operation break line) and only after join the lines or adjust them automatically.

B-) The user did not make the super positioning of the nodes: in the digitization the polygon is kept open, or a line does not reach or exceeds the interception point. Example:
 

For these cases the automatic adjustment of the nodes could not be sufficient to close these polygons, one should then perform the manual edition, joining or approximating the lines, or even increasing the threshold of automatic adjustment.
 

Automatic Topology

A-) The user exceeded the limit of the intersection - since a line can be automatically broken, a short line could be left over and should thus be manually eliminated (operation delete line), in case the digitizing step is smaller than the left over line length. Such error demands the operator attention during digitization.

B-) The user did not define correctly the limits between polygons: in the digitalization the lines can be superimposed or you may leave a gap between them, when a message of invalid incorporation is presented.

Superimposition - since nodes were inserted the error will only be detected during the polygon generation, and should be fixed through the manual edition.

Gap - this error cannot be detected by the editing operations of the system, and is related to the precision of the operator during digitization.


Adjustments

The adjustment step consists in making the arcs having their ends, that is nodes, connected. Usually we use the automatic adjustment when the errors are small or are within the tolerance limits defined by the user. When greater errors are inserted it becomes necessary to perform the manual adjustments ( see more details about the adjustment tools).

Polygonalization

    Once all the lines were adjusted, we must create the topology by performing the polygonalization step, when the topological relationship between the polygons will be created or updated. See more in how to digitize.

See also:

How to calibrate the Digitizing Table
How to digitize a Map ?
How to Create an IL for Digitizing ?

Vector Editing