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Tutorial 4: Working with Tables in ArcMap: Colouring Layers According to Attribute Data Tutorial content: 4.1. How can I assign colour/ a legend based on information in the attribute layer?

a) When should I use a unique colour? b) When should I use a graduated colour?

Open your saved tutorial map project. 4.1. Colouring Layers Based on Attribute Data So far we have looked at customising layers with single symbol legend types, where every feature on a layer is displayed in the same way. Often we need to differentiate between different features, for example being able to distinguish one land use type from another. It is possible to assign colours in ArcMap based on information recorded in the data layer’s attribute table. We will look at how to add data to a layer’s attribute table in tutorial 5. Add the two shapefiles: drift_geology and solid_geology from the folder Edina Geology Data in the GIS Tutorial Data Folder on your U:drive. Both layers are composed of polygons and at the moment all polygons are displayed as the same solid colour. Right click on the solid_geology layer in the table of contents and select Open Attribute Table from the drop down menu.

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A new window will open with a table displaying the attribute data for the ‘solid_geology’ layer.

Each row of data in the table corresponds to a polygon in the map display. To demonstrate this, move the attribute window to the top of the screen by clicking on the bar at the top of the window, holding down and dragging. Use the ‘Pan’ tool in the ‘Tools’ toolbar to move the map display so that the ‘solid_geology’ layer is visible (as in the screen capture below).

OS map data © Crown Copyright/database right 2009. An Ordnance Survey/EDINA supplied service.  

OS map data © Crown Copyright/database right 2009. An Ordnance Survey/EDINA supplied service.  

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Next, we will click on one of the rows in the attribute table to highlight it. To do this, click on any of the squares to the left of the data column headed ‘FID’ The row will be highlighted in blue and the corresponding polygon in the map display will also be highlighted

The data in the attribute table can be used to assign colours to each polygon on the ‘solid_geology’ layer. In this case we are going to assign colour based on the geology type, which is listed in the table under the column headed ‘LEX_D’. Close the attribute table of “solid_geology” window by clicking on the red button with the white cross in the top right corner of the window. Double click on the layer name solid_geology in the table contents to open the ‘Layer Properties’ window. Select the ‘Symbology’ tab. At the moment the option under ‘Show:’ is set to ‘Features’ and ‘Single Symbol’. a) We are going to use a unique value legend to colour the geology data. Unique value is appropriate in this case because we are dealing with data where there is no quantitative relationship. Another example where a unique value would be appropriate is a data layer showing different land use types.

OS map data © Crown Copyright/database right 2009. An Ordnance Survey/EDINA supplied service.  

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Select the option ‘Categories' the default option Unique values will also be selected. Under ‘Value Field’, ‘LEX_D’ the column in the attribute table that details the geology type is already selected. Other columns from the attribute table are listed as options in the text box and can be accessed using the arrow on the right. In this case we want to accept the default option: LEX_D. Under symbol, un-tick the box next to the coloured polygon. Then click on the button to Add All Values. All the different geology types will be added to Layer Properties window and assigned a unique colour. To change the default ‘Color Ramp’ that has been used to colour the layer click on the arrow to the right of the ‘Color Ramp’ window. Select a colour ramp from the drop down menu. Click Apply to preview the colour ramp on screen. Colours can also be picked individually for each geology type by double clicking on the coloured symbols and selecting from the ‘Symbol Selector’. Click OK when you are happy with your selection.

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Try colouring the layer ‘drift_geology’ by geology type in the same way.

So far you have looked at using a ‘unique value’ to colour the geology data. b) Another option in ArcMap is to use a ‘Graduated Colour’ or ‘Graduate Symbols’. Graduated colour should be used for layers where there is a quantitative relationship between the data, for example the illustrate height above sea level or to show the population density in different geographic areas. Graduated colours are particularly useful for contour plans or when working with census data. To apply a ‘Graduated colour or symbol’ to your data layer, double click on the layer name in the table contents to open the ‘Layer Properties’ window. Select the ‘Symbology’ tab. Under ‘Show:’ select the option ‘Quantities’, then ‘Graduated Color’. In the ‘Value:’ text box under ‘Fields:’ select an appropriate column from the layer’s attribute table to base the graduated colour on. Then click OK.

OS map data © Crown Copyright/database right 2009. An Ordnance Survey/EDINA supplied service.  

OS map data © Crown Copyright/database right 2009. An Ordnance Survey/EDINA supplied service.