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Section Three


Section Three: Creating Thematic Maps

Part One: Creating Thematic Maps



GIS logoThematic maps are maps which are of one particular variable. This may be, for example, a map showing the population of each ward. Producing professional thematic maps is one of MapInfo's great strengths.

Creating thematic maps


Map > Create Thematic Map

MapInfo has a user friendly three stage wizard to create thematic maps.

Mapping Wizard The Create Thematic Map wizard may also be accessed by choosing the Create Thematic Layer option from stage 3, Analyse, of the Mapping Wizard Tool.
Step One
  • Step One: Choosing the type of map and the colour scheme.
  • From the selection of buttons down the left hand side of the wizard, select the type of map you want. Different types of map are appropriate for different types of data. The top button, ranges, is selected by default. We will leave this selected for our example, as we are going to make maps of the different ethnic groups in Bradford in terms of their percentage of the population at ward level.
  • GIS logoRanged maps are appropriate for discrete variables, i.e. variables for which there is one value for a whole spatial unit and the value for adjacent spatial units is different. Pie chart maps are appropriate for representing multiple varibales, such as if we had chosen to represent all the ethnic groups as a percentage of the population at ward level on just one map. Grid maps are appropriate for mapping continuous surfaces, i.e. ones which change gradually, such as population surfaces interpolted from other variables. 3D maps are more appropriate when the data will be viewed on screen than when the data are to be printed out.
  • Also available are bar chart and pie chart maps, which produce a graph in each geographical area (such as a ward), graduated symbol maps, which show the count of the data for the chosen variable within each geographical area as a symbol, the size of which varies according to the count, dot density maps, where the density increases according to the count, individual maps, where each geographical area has its own colour, and grid maps, which show variables as a continuous surface as one would see with a raster dataset. These, however, may be difficult to interpret.
  • Leave the range button selected. From the list in the centre, choose a colour scheme for your map. This is previewed in the panel on the right.
  • HintDon't worry if you can't find a colour scheme that is exactly what you want. These colour schemes are just a starting point, and you may change individual colours later.
  • When you are happy with your selection, click next.
Step Two
  • Step Two: Select a Table and a Field
  • GIS logoMapInfo tables are referenced like other databases. In each table, rows are called "records" and columns are called "fields". This means that in MapInfo, each field represents a variable.
  • Choose the table which contains the variable you wish to map from the drop down list of all open tables.
  • From that table, choose the variable you wish to map from the drop down list Field. This is one reason why your variables need self explanatory names, so that you can be sure you are using the right one.
  • At the bottom of the Field list is Expression... This command is used for mapping mathematical functions of other variables. We will make a map of the white population of Bradford as a percentage of the total population. Select Expression... now.
  • A dialog box appears. The panel on the left invites you to type an expression, but it is easier to use the drop down menus on the right until you are familiar with MapInfo.
  • From the Columns menu, choose the column which stores the white population. The name of this column appears in the panel on the left.
  • Type a / after this. From the Columns menu, choose the column which stores to the total populaion. When the appears in the panel, put parentheses () around everything you have so far.
  • Type * 100 at the end. Your panel should now read something like this: (white/total)*100
  • HintMathematical operators such as add, subtract, multiply and divide are available in the drop down Operators menu, but it is normally quicker just to type them. Do not worry about the functions menu for now.
  • When creating more complex expression, it is useful to click verify to make sure your syntax is correct, for example, that you have included parentheses in the right places. Do this now. You will get a dialog box telling you your syntax is correct. Click OK, and then click OK in the Expression... window to return to the step two window.
  • It is usual to tick the ignore zeroes and blanks box, especially for 1991 census data which include a Shipping ward. Click next.
Step Three
  • Step Three: Customisation.
  • A preview of your ranges and colour scheme appears. However, it is likely that this will require changes to produce the final map. Alterations may be made using the buttons on the right.
Ranges
  • Ranges: This allows you to alter the number of categories you have, and the ranges included in these. The default is equal count, i.e. an equal number of wards within each category. However, for some applications it may be necessary to alter the ranges, for example, to be directly comparable to other maps with different input data. Click OK when you are happy.
  • HintAltering the ranges to your own custom ranges is very useful in order to make certain areas to stand out, for example, to illustrate the point you are making in a report.
Styles
  • Styles: This allows you to change the colours of each category, if you wish. Select the category by clicking its button (which shows its current colour). It is advisable to just change the foreground colour and leave everything else as is. It is also advisable to leave all the other settings in the Styles dialog as their defaults.
  • If you have created a new colour scheme which you wish to use again, choose Save As in the Template box at the bottom left of the Step 3 window. Give it a self explanatory name and do not overwrite any of the default templates.
  • HintMake sure that there is sufficient contrast between the colours for all your categories, especially as colours on some printers show up slightly differently to on the screen.
Legends
  • Legend: While your ranges and style may not require any customisation, your legend almost certainly will. MapInfo gives your legend a default title of the table name and field or expression to be mapped. Change this to a name which actually makes sense and is grammatically correct, such as The White Population of Bradford as a Percentage of the Total. In some cases a shorter title is OK. A subtitle may also be added. The fonts of these may be changed, if you wish.
  • Leave the Into Window choice as New Legend Window. The alternative is no legend. It is poor practice and, in many cases, pointless, to have a map without a legend.
  • Choose the format of the range labels to be displayed. In most cases it is not necessary to make changes. You may also change the font if you wish.
  • HintDo not change the values in the range labels as this will not change the ranges themselves, and your map legend will not be correct. Use the Ranges button to change the range values.
  • Choose whether or not you want to show the record count, i.e. the number of wards that fall into each category. This appears as a figure in brackets at the end of each range in the legend. Normally is it not necessary to show this, especially when the equal count method has been used to assign the ranges. The fewer the areas on the map, the less necessary it is. However, if you do choose to show the record counts, be sure to explain what these figures are, for example, using the subtitle. Click OK when you are happy.
  • Click OK to display your map. The map will appear in the New Mapper which you opened, and the legend will appear in a new window. You may need to restore the map window if it is maximised in order to see the legend.
Bradford Population 1991Legend

HintIf you are unhappy with the ranges, styles or legend select Map > Modify Thematic Map, after making sure the map you wish to modify is the active window. This brings up a dialog box which is the same as step three of the Create Thematic Map wizard, from which you can make changes. If you wish to change something from steps one or two you will have to start again.

Layer control

Once you have made your map, an important feature of MapInfo is Map > Layer Control.

Layer Control GIS logo A map in a GIS may be made up of several layers. For example, a map of the British Isles may have a region layer showing the outline of the British Isles, a line layer showing major roads, a point layer showing major settlements and an attribute layer showing text labels for the points and lines. Layer control allows the user to add, remove, reorder and customise layers.
  • Cosmetic Layer: This is the topmost, transparent, layer, which cannot be moved or deleted. It is here that titles etc may be added.
From left to right, the checkboxes for each layer represent:
  • Visibility: Can the layer be viewed in the current map window? If not, it may mean that the table containing the data for that layer is not open.
  • Editable: This must be checked in order to be able to make changes to a layer.
  • Selectable: Objects must be selected if they are to be used in analysis such as queries (Section Four, Part One). More than one layer may be selectable at the same time. If a layer is editable it is automatically selectable.
  • Auto-label: Labels the layer with the first column of the data in the browser, i.e. with the leftmost column of the data table. This feature is unchecked by default.
  • HintThe first column in the table is normally the census area code. Labelling the map with this is often highly unhelpful. Make sure auto-label is checked for the layer you wish to add labels to, then choose the Label button at the right of the dialog box. From the Label With... drop down menu, choose the column you wish to label your features with. For census data, zone name is far more useful than zone ID, the default. This dialog box also allows you to choose the font and size for your labels using the Style (Aa) button, and to choose where you want the labels relative to the feature they are labelling. Click OK when you are happy. Click OK again to exit Layer Control. Labels may then be dragged into position and rotated if necessary. Individual labels may be altered by double clicking on them, for example, to split a ward name into two lines of text. However, to make changes to all the labels you will need to return to the Layer Control dialog box. The Label tool on the main toolbar may be used to add individual labels.
  • The Display button brings up a dialog box allowing the user to override the default settings of the map composition, such as colour schemes and fonts. These changes may be applied only to the current session, or changed for future use.
  • Use the Add button to add a new data layer to the map composition, and the Remove button to remove an existing layer.
  • Click OK to exit Layer Control. The selected changes will then be applied.


On to Section Three, Part Two: Creating Graphs



Section Three
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