Each feature within a GIS has appropriate attribute data attached to it. This data can contain information such as, in the case of a city, its name, size of population and other relevant details.
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All GIS store data in a format which is commonly referred to as an attribute table. Each layer contains an attribute table which is similar to a spreadsheet layout. Within this table the user can put as much or as little information as is required for the layer’s requirements. Like a spreadsheet the data is stored in a series of rows and columns. Each row represents a single record and each column represents a field within the table. Some tables only contain a few fields with a limited number of columns, whilst other tables, say containing crime statistics, can have thousands of records and numerous fields.

Information within the attribute table is linked to the point, line or polygon within the map layer. Clicking on one or more features in the map layer will also highlight the same features in the attribute table if this has been made visible. Similarly, clicking on one or more rows in the attribute table will highlight these features in the map layer.  

The fact that the map layer and the attribute table are linked enables us to be able to ask questions about the data contained within the table and show the results on the map layer. We could query our data to find, for example, the largest cities or the number of airports within a region or where the population is over a certain level. In fact, the modern GIS enables us to ask an almost unlimited number of questions about the data contained within the layers of the map.

As well as being able to highlight features on our map layer we also have the ability to create thematic maps based on the data within the attribute table. Sometimes, it is easier to understand the underlying data within the layer when we create a thematic representation of that information. For instance, population by region can be easily be shown by using a thematic representation.

As well as being able to ask questions about data within a single layer it also possible to create a query which covers multiple layers. To do this we can use the SQL feature, which stands for standard query language, available within the GIS.

Some GIS software such as MapInfo Pro and ArcGIS Pro have always had features to enable users to create quite sophisticated queries from layers within their proprietary software. Until recently, QGIS, which is open source, was not able to match this level of sophistication. Now however QGIS has a feature which can enable users to create sophisticated queries using the virtual layer facility.

Desktop GIS now has many features which enable the most complex questions to be asked about the layers within a project. The visual results of such queries can often give a greater insight than the equivalent spreadsheet bar or pie chart can achieve.   

 Desktop geographical information systems use scale to represent the relationship between the size of elements within the map and the related areas within the real world. In fact it is very similar to the methods used with paper maps. However, paper maps are produced at fixed scales to meet different requirements. This contrasts with the flexibility of a desktop GIS which can produce an infinite variety of maps by adjusting the scale to meet specific requirements.

A ramblers paper map would have a scale which relates more closely to a specific area of interest. A map used for travelling over larger distances by car for example would have a different scale. The only way someone could take a closer look at a detailed paper map would be to use a magnifying glass which would have no impact on the scale of the map. However, with a desktop GIS  there is the capability of zooming into areas of interest which then  automatically adjusts the map scale.

So with a desktop GIS the zoom feature enables users to view maps at a variety of scales. In the following screen shot the map is shown at a world view of 1:186,760,885.  

In the next screen shot the scale has been changed to 1:13,877,198 as we zoom in to take a closer look at an area including parts of the United States and South America.   

The effect of zooming in shows that in the above map is now nearly fourteen million times smaller than actual size. The map now has a scale value of 1:13,877,198. The zoomed in area of the map within the GIS has in other words a ratio 1:13,877,198. This represents one unit on the map as almost fourteen million times smaller than in the real world.

We can also indicate the scale of a map by adding a scale bar to maps that are printed. We can set the scale to the desired measurement which could be either metric or imperial. In the following screen shot we have set the scale in kilometers.

So as we zoom into a map we can obtain a closer view of the map’s features. At different scales we can obtain just the right amount of information for our specific requirements. The ability to create a variety of maps from a single map source is made possible by the flexible scaling features within a modern GIS system. Maps can then be printed at a suitable scale to meet the user’s requirements.

Prior to the availability of computerised mapping facilities paper maps were often used in conjunction with acetate sheets in all forms of mapping applications. At that time the latest paper map of a county or region would have been an expensive outlay. Within the UK, local authorities obtained their paper maps from Ordnance Survey often buying updates several times a year at a cost of thousands of pounds.   

Because the maps were expensive it made sense to ensure that they were well looked after. Sometimes this was achieved by laminating the maps to stop them being damaged by accidental spillages. If additional information was needed to be added to these base printed maps, acetate sheets were used. These acetates were used to mark out, for example, property boundaries or crime incidents. In this way several layers of information could be added without causing any damage to the printed base map.

Today, the modern desktop gis uses the same principal to separate the base map from other information stored within the system. The layer system enables the user to break information down into specific areas of interest. For instance in a map of Alaska all the airports are contained within a separate layer within the gis. This information on its own would not make a great deal of sense without the underlying base map as the following graphic shows.

Without the underlying Alaskan base map the airport layer does not make a great deal of sense as it is not related to anything meaningful in isolation. In the next graphic the airport layer makes sense as it shown with the relevant base map.

Maps can have many layers within a project. Layers can be present within the map without being visible. This enables specific areas in the project to be analysed by the map maker. Often a map may contain very many layers and it this case it makes sense to group related layers. In grouping layers you can then more easily manage whether the group is displayed or not. The group of layers can then be turned on or off together rather than manually turning off the related individual layers one at a time.

Some gis systems, such as MapInfo allow placement of points, lines and polygons within the same layer. However, whilst this is possible within MapInfo, if you need to transfer to a different gis you may find this creates problems. Both QGIS and ArcGIS have a layering system which only allows for one type (point, line or polygon) to display within a layer. Transferring data to QGIS from MapInfo with layers containing a mix of types will result in QGIS creating additional layers during the conversion process. Alternatively, MapInfo can convert layers to a shape file format which both QGIS and ArcGIS can then use.

Layers can be in a vector or raster format. Vector layers are either points, lines or polygons which can be in an x and y, or latitude and longitude format. Raster data consists of a series of dots and is often used to represent for example, ocean depths or variations in vegetation growth. In the next screen shot a raster layer has been added to the Alaskan map.

Layer order is important when displaying data within the map. If the layers are not ordered correctly some layers may not display correctly, if at all. To change the layer order you only need to click on the layer within the layer control section, hold the mouse down and then move the layer up or down to achieve the correct sequence.