"BEST-GIS" ESPRIT/ESSI Project n. 21580


6. List of key GIS operations  

The development of this list is based on the assumption that better informed users can make better decisions regarding their needs, and can then better describe those needs to GIS vendors and consultants during the selection and implementation phases. 
It is often difficult for a person to judge the potential value of a tool which he/she has never used.    
Because new GIS users often do not possess the experience (and thus the information) necessary to objectively judge their needs, this section provides a sample of the common GIS functions (operations) which the user should keep in mind. Some operations will apply directly to the user's intended application, while others may be unnecessary.   
The following list contains the more common GIS operations, along with  a short explanation:   

  • if it is a command, a function (two or more commands) or a process (two or more functions)  
  • the type of data/circumstance on which it commonly work, and  
  • how the item is most frequently activated. 

Based on the requirements stated with the help of the checklist in chapter 5, the user should select the operations which he/she finds potentially interesting or useful to the intended GIS application.    
These would presumably form the basis of a statement of user needs, to be presented to the vendors competing for the GIS contract during a system benchmark test. During such test, the vendor should demonstrate not only that the function is supported (yes or no) rather how it is implemented, what resources are required in order to realize the operation, and how does the user interact with the operation (the user interface perspective).   
 

6.1 Project set-up  

  • set parameters 
  • set minimum spatial resolution of each feature 
  • set coordinate system, map output scale, etc. 

Project set-up is normally comprised of a set of commands used to initialise the GIS software, the new or existing database and the viewing parameters.    
  
  
6.2  Data entry  

6.2.1 Map digitising  

Map digitising is the process of passing the location of geometrical features (points and lines) from analogue (paper map) to digital format, creating a vector cartographic database. This is normally accomplished using a mouse-like cursor to follow linework and to register the key points used to represent each feature. Most GIS accommodate this process via a digitising subsystem (often a menu of several commands).   
 

6.2.2 Map scanning  

Map scanning is the process of digitising geometrical features of a paper map using a black and white or colour scanner, producing a database in raster (cellular, bitmap) format. This is normally a simple process of setting scanner parameters (resolution, colours) and then passing the map through the scanner/reader. The process is often used as a preliminary bulk  data input stage, so that the raster data can be vectorised using semiautomatic line-following software.   
 

6.2.3 Bulk/batch loading  

Bulk or batch loading is the function whereby a large quantity of data are read from an external source, as in the case of a file of x,y,z coordinates in ASCII format. This function may be a single command  taking several parameters indicating the file format or type.   
 

6.2.4 Assignment of basic attributes to each feature (ID of point, line, area, volume)  

Attribute assignment is the tagging of each geometric feature in the digital cartographic dataset, usually accomplished during the digitisation of the feature, using the numeric keys on the digitising cursor to enter unique numbers or codes (Ids) . The process is normally integrated in the data collection module of the GIS (see Chapter 7).   
   

6.3  Data conversion  

6.3.1 Raster-to-Vector   

Conversion from raster to vector-format of spatial data (usually derived from scanning, see section 1.2) involves either a single batch process, or a semiautomatic operation of line-following. When the input raster has a low resolution (i.e. satellite imagery), a fully topologically and aesthetically correct conversion is achieved by very few GIS (see Chapter 7).   
 

6.3.2 Vector-to-Raster  

The converse of 2.1, vector to raster conversion converts the geometric features of a database to a representation using a raster grid of (normally) regular cells or pixels. The process is most often automated as a batch process on a single vector data file, and the user must specify the resolution and extent (number of rows and columns) of the grid, and some cell encoding preferences.   
   
  
6.4  Data validation  

6.4.1 Identification/correction of topological errors  

Identification and correction of topological errors in vector map database features--an important quality control process-- is a cyclical process involving several commands or functions. Errors (overshoots, undershoots, open polygons) can be visualised and categorised, and then may be edited in a batch process or (normally) manually.  

 
  


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Two are the interesting issues that can be seen in this picture. First, the handling of background environment (here, vector data, symbolised by different colours). Second, in terms of data validation, the identification of possible topological errors: in these views, we highlighted (with yellow circles) the areas on the current edit "coverage" (white vectors) that the GIS informs us (by the small squares) that a line is open (not closed, not defining a polygon). So we can examine if there are any unwanted overshoots, undershoots, etc. & afterwards, we can correct them.

  

6.4.2 Identification and correction of tabular data format  

Often tabular (attribute) databases already exist in relational database management systems (RDBMS), and  they are to be "connected" to the geometric part of the GIS, however the data are often not in the desired format or structure. Tables may need to be joined or split, and new relations between tables formed, according to the desired geographic model. These actions would be executed using several commands within the database management system.   
 

6.5  Data visualisation  

6.5.1 Zoom/Pan/change view  

Zooming is the process of magnifying or reducing the scale of a map or image displayed on the monitor.    
Panning is the process of changing the position at which the view is displayed, without modifying the scale  (see chapter 7).   
  


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Interesting subjects are how a GIS accomplishes feature filtering (we see here the checkboxes), feature symbolization, zooming & panning (the 2nd view on the right is a zoomed part of the 1st), attribute table editing (see the table lower left where the user can directly intervene), scaling (notice the small frame upper right indicating the scale of the view, where we can fill in a desired scale & and the view will change automatically), etc.

  

6.5.2 Redraw/refresh entire display  

The process of editing and erasing graphic elements on the monitor can often lead to undesirable residual "ghost" graphics which contaminate the view; thus the user would ordinarily refresh the screen with some  frequency, using a single command or icon. This function also accompanies 4.1, as an automatic process, because the screen image must be redrawn after scales changes or displacements using zoom and pan.   
 

6.5.3 Feature symbolisation (simple colour or symbol changes)  

The process of associating certain geographic or label features with selected patterns, colours etc. (i.e. roads may be coloured according to their class or traffic density). This process is normally executed from a symbol editing menu and may also require the definition/editing of reference (look-up) tables.   
  
  


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Elevation contour lines for Rhodes island, using two different methods of data visualisation. A GIS must provide almost unlimited choices of feature symbolization, permitting user to select between various classification methods, ready color palettes & ramps, etc.

  

6.5.4 Feature filtering (hide a layer or feature, show another)  

Feature filtering involves the selection of the graphic elements that the user wishes to visualize at a certain moment. Some GIS offer a menu of possible and active features, while others require the user to remember layer or feature names as part of the command sequence.   
 

6.5.5 Management of background images  

The process of displaying and managing on the same view vector objects (points, lines, areas) in the foreground and georeferenced raster imagery in the background (see Chapter 7).   
  


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Beyond the strength of a "windowed" interface in relevant to the easiness of making different aspects & zooms (as you can move, resize, minimize etc. the various parts of your visual environment), in this picture we can see the accurate overlaying of  georeferenced raster background images (here in colored & opaque texture) with vector elements. In this way, we set the proper environment, par example, for "on screen digitising". 

  
  
  

 
 

A screen display showing a   scanned and georelated paper map and the framing for the plotted paper outputs of the planning study (regional map, scale 1:25.000).

  
 

The same as above, pointing out from one of the previous frames some outstanding points of the old railway (regional map, scale 1:25.000).

  
 

Merge of a map with an aerial photo, to better identify the points of interest: crossings, underground passages (regional map, scale 1:25.000).

  
 

A zoom out of the above.

Two frames for the project, cut down from the overall aerial coverage. Some vector information is present as well.

  
  

6.6  Map database management  

6.6.1 Joining of map sheets/tiles (edgematching, etc.)  

Many GIS packages allow the segmentation of large spatial databases into tiles or pages, for reasons of optimal memory management or user convenience during editing. These tiles can become cumbersome artifacts during analysis and, thus, the GIS normally allows for the stitching or joining of tiles as well as the general concatenation of map data files (e.g. to join a new urbanisation to a city map). The process often is a manual one, requiring considerable user intervention to precisely guide the joining of features at tile boundaries.   
  


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Here we have two examples of using a GIS for analyzing data, aiming to the production of thematic maps & finally, for decision making (both refer to Peristeri municipality - Athens). The first shows the areas that are well served by public transportation (buffers) & the second, the urban areas that establishment of amusement uses must be forbidden (circles that define 200 m range from schools).

  

6.6.2 Rectification/conflation of layers (i.e. satellite image with vector map)  

It is a common occurrence that various data layers (e.g. soils, roads, land-use) of the GIS database come from disparate sources, scales, coordinate systems, etc., making rectification of one layer with another base layer a necessary operation. This operation often requires substantial user intervention, as control points must be selected and/or introduced and then several slight modifications may be necessary, especially when one layer is to be aligned (conflated) to precisely match another.   
  


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"This is an example of displaying in the same view vector elements in the foreground and georeferencd raster imagery in the background. 
The background is a satelite composite photo of Salamina island (near Piraeus - Greece), an image that has been already rectified (aligned) with our vector data (see pictures Andip 6 & 7)".

  

6.6.3 Georeferencing (raster and vector data)  

Georeferencing is the process of associating known locations in the real world to the corresponding locations on the cartographic dataset. A subset of these locations, normally of high accuracy, may serve as "control points" to assist in the interpolation of other points in the dataset.   
  
  


Click on the preview of each image in order  to see a larger version  of the photo 
Click on the preview of each image in order  to see a larger version  of the photo 

In these two pictures we can examine a method for raster data rectification. In the lower left window of each one, we have the red vector "coverage" (island of Salamina, near Piraeus), that is established in a certain projection system & its elements have "real" coordinates. The black & white (upper of it) picture is a raster (scanned) image of the same area & we want to transform it from its ”paperā coordinates to the real ones. Adjusting frames in these two small windows, we define the areas we want to overlay & then we put the first 4 links (green arrows in the right window of the 1st picture). After the first registration (which brings us to the 2nd picture), we put more links to achieve better transformation, trying also to have low RMS values, by deleting and keeping the most proper ones (see the two upper windows).

  

6.6.4 Projection change  

Various input maps might have different projections, a mathematical treatment of the coordinates to adjust for Earth curvature while preserving angles, areas, or distance. These projections may be changed to suit a particular project or to be consistent with other data layers, and normally involves a single or few batch commands on the cartographic file in question.   
  

6.7  Attribute data management (assuming relational database manager)  

6.7.1 Link (join) basic attribute to main database  

Often during the digitisation process a single unique ID number is associated with each feature (e.g. each line segment of a road network), the feature?s initial attribute. That attribute then must be connected (joined) to the main attribute database to provide graphic query capability of the type "what are the characteristics of this <select with mouse> object? This linking process is normally a function of the database management system chosen.   
  


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Selecting & querying about  features must be easy & flexible, as it is a very common, everyday task in using a GIS. We see in the picture a polygon (yellow coloured) from a query (area > 10000000 & type = 3), by filling a form (lower right). Selection, is critical to can be done (as here) simply by pointing on a feature in the view window or in the attribute table window & the selection colour (here yellow) reflects in both windows the same time. We can also see in a table form (lower center) the identification results of a feature we selected just pointing on it, using a proper button (i).

  

6.7.2 Establish more complex relations  

Relational database management systems (RDBMS) are normally used within GIS packages, because of their ability to define complex relations between key feature attributes; this allows for quite complex models to be    constructed (see section 7).    
 

6.7.3 Establish connections (SQL, middleware) to secondary databases/systems  

GIS is often introduced to an organization which already has in place an information system and where existing (legacy) systems hold data which is valuable to GIS applications. It is often a nontrivial task to connect the GIS to the wider system, using middleware applications to translate from one to another; normally an expert creates the application and then the end user executes a command or set of commands to connect to and utilize the external system.   
 

6.8  Data processing/analysis  

6.8.1 Create/save database views (via RESELECT)  

Databases views is technical terminology for the issue of queries to the GIS of the type "show me the segments of the selected highway which have not been maintained in the last 36 months".  These segments could then be saved as a selected set, called something like "priority segments", for later processing. This process is normally based on the inherent capabilities of the relational database manager (see section 7).   
 

6.8.2 Proximity analysis (buffer, distance calculation)  

A common spatial analytic procedure is the calculation of features within a certain distance (often radius) of another feature set, as in "show the parcels within 100 metres of the selected highway". This process can be a direct query which results in the selected elements (or grid-cells) being highlighted in another colour, or it may involve several commands to first define the proximity polygon and then to query its intersection with other features.   
  


Click on the preview in order to see a larger  version of this photo 


Click on the preview in order to see a larger  version of this photo

Here we have two examples of using a GIS for analyzing data, aiming to the production of thematic maps & finally, for decision making (both refer to Peristeri municipality - Athens). The first shows the areas that are well served by public transportation (buffers) & the second, the urban areas that establishment of amusement uses must be forbidden (circles that define 200 m range from schools).   

  

6.8.3 Spatial joins (overlay)  

Still considered the GIS's fundamental analytical operation, overlay involves calculating the spatial coincidence or intersection of the features of two or more layers, producing a resultant layer and the associated joined attribute tables (see section 7).    
 

6.8.4 Network analysis (optimal routes, allocation of resources)  

GIS which utilise topological vector data structures to preserve connectivity often include the ability to calculate shortest distance and other network trace analyses. These are useful for transportation applications such asdistribution, routing, navigation, etc. as well as the management of resources over the network, such as gas, electricity and telephony. The process is quite complex; first the user must create the network model, then set the proper attributes (speed limits, turn restrictions, flow impedances, etc., and then must run the analysis routines and display the results.   
 

6.8.5 Raster analyses (map algebra)  

Many GIS allow the processing of raster data, taken normally from remotely sensed imagery. Among the available raster analysis is normally the ability to overlay raster maps for processing using "map algebra". Often the GIS will have a separate subsystem devoted to this processing, which uses its own language and methods.   
 

6.8.6 Generalisation/smoothing/dissolve  

Among the most widely used cartographic functions are those designed to simplify and generalise vector    
cartography, reducing the points necessary to represent features, merging similar neighbouring areas, etc. This is usually implemented as a set of singular commands, and a word of caution is in order regarding their appropriate use.   
 

6.8.7 Digital terrain modelling (incl. simple creation of 3-D views)  

This is the complex process of building a three-dimensional representation of a surface, given a set of elevation points (z- values). The representation normally serves only for improved visualisation of an area of interest.    
The GIS often includes a subsystem for importing, manipulating and viewing these models (see section 7).   
  


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This is an example of 3D view creation by a GIS (Piraeus & West Attica).   
 

  

6.8.8 Secondary analyses (on DTMs) 

  • drainage, flow 
  • slope/aspect 
  • other... 

6.9  Output: map production /reports  

6.9.1 Generate summary statistics  

Among the tabular (non-cartographic) products which may be generated by the GIS, summary statistics (i.e.  sum, mean, standard deviation, etc) of key attributes (e.g. area of polygons) may be calculated. These are often single commands, issued from within the GIS?s database manager.   
  


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In this picture we see a simple form of data analysis (center left window, where land use polygons are color-classified), a friendly environment where the user can set his/hers preferences for selecting and querying among features (upper left window) & an interesting way of data visualization, using various graphic elements, as chart bars, pies, etc. (right section of the screen).

  

6.9.2 Generate text report from attribute database  

The database management system (DBMS) may also allow the generation of summary text reports from the attribute tables. This is an important output, given that often the GIS analyses graphic features, but then reports tabular results. These are also generated from within the DBMS, and thus will vary from system to system.   
 

6.9.3 Generate simple map  

Most commonly a GIS produces cartographic output in the form of a simple map. The system may ask for several graphic parameters to be specified, and then the drawing appears with a single icon or command.   
 

6.9.4 Generate complex graphic product (map composition)  

The combination of map layers, tables, legends, etc. in order to produce a complex graphic output (i.e. a poster) which includes information from many sources.  This process is often executed from within an output subsystem, and involves composition and design, complex symbolisation from attribute tables, and knowledge of plotting parameters such as scale, print size, etc.   
  


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Largest area of city of Rhodes, in a map composition generated from a macro language file (upper left window) & visualised in a window as a whole (lower right) & in several zoomed parts (main city upper center & a legend part upper right). The map can be converted to a graphic file or be plotted directly. Some relevant issues critical for the end-user : Are the methods for compositioning a map (text commands &/or tools) convenient & easy-to-learn? Are there enough tools & user interface features for legend creation? Are the colours on the screen similar to the plotted ones? 

  
  



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