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written by Nigel
James
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MAPINFO How to... | |
No. 16: Using the Terrain 5 gridded elevation dataset | |
(References to tables, folders etc., are not applicable outside the Map Room, so if you are located elsewhere, you should substitute your own) | |
The Terrain 5 Gridded Elevation dataset is a 5x5 min gridded dataset
of elevations in metres (including those below sea level). This is a world-wide
dataset consisting of around 9 million points, each having the elevation
in metres as an attribute. |
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Mapping relief | |
You can use the Grid Thematic mapping function to produce
a map using colour layering. The height data is in the Height attribute).
For step by step instructions on how to use the Grid Thematic method,
see
MapInfo How to guide no 7: Create a Grid Thematic Map. If any of the
following steps are not clear then have a look at How
to no 7 for more details.
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1. Assuming you have created a new table of the area you
require, open this table. If the full dataset table is still open then
close it (File>Close table and select theTerrain 5 table).
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2. Open the Map menu and click Create Thematic Map. | |
3. In the Step 1 of 3 dialog, click Grid type. | |
4. Click Grid default in the Template name list and then Next. | |
5. You will now see the Step 2 of 3 dialog. In the Select Table and Field boxes, the Table should be your copy of the terrain5 data and the field should be datavalue (i.e.. the field containing the height data). Leave Ignore zeros or blanks unchecked. | |
6. In the Grid Options box, Select
table to clip against is where you can choose a table of
region objects to clip to, e.g: if your map is of a coastal area and you
have a layer containing the land area (such as country polygons) then
this can be used to clip the relief data. Clipping the data means that
it will only display within the region objects, so the sea will be uncoloured.
NB: Clipping can have a significant impact on the time taken to display
the layer. |
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7. The Grid File Name should be a file in your user folder, so click the Browse... button and select your user folder. The filename should have a .mig extension. | |
8. You will now see a preview of the legend. At this stage it will be multicoloured, but if you wish to change the colour style that can be done later, so leave it as it is for now. Click OK to display the thematic. | |
9. The points will still be displayed, but you can hide these by selecting your grid table in the Layer Control (it will be above the thematic layer, so select the right one!) and clicking the Display button. Click the Style Override checkbox and then the large button with a symbol on it. In the Symbol drop-down list, click N for none and OK. OK each dialog including the layer control to redisplay the map. | |
10. You can now experiment with different settings to change the style of the map. This steps are detailed in MapInfo How to no. 7. Changes you can make include the grid size and search radius to change the way values are interpolated. | |
11. For a relief map, you may wish to change the colour range, say from dark green to light yellow and to do this, open the Map menu and click Modify Thematic Map. | |
12. In the Modify Thematic Map dialog click Styles. You will now see the Grid Appearance dialog. You can change each inflection manually, by double-clicking the colour box, but it is much easier to let mapInfo do this for you. | |
13. Start by reducing the Number of Inflections to 2. Now double-click the top colour box in the Percentage list and change it to the colour for the lowest height. Next, double click the second clour box and change it to the colour needed for the highest point. You will see the new colour range in the vertical colour bar. If this looks OK, increase the number of inflections again (the more inflections the smoother the colour transition). Check that each inflect relects the percentage value you need, e.g: for a 5-value range they should be 0%, 25%, 50 %, 75% 100%. Alternatively change the Method to Custom value and you can enter numerical values (i.e: height) for each inflection. | |
14. The Brightness and Contrast sliders can be adjusted to change the appearance, but generally they are OK left as they are. | |
15. Click OK to see the new appearance. If you wish to make further changes, just open the Map menu and click Modify Thematic again. | |
17. The thematic layer can be saved as a separate layer (File>Save Copy as) and selecting the grid layer (not the datapoints layer). Save it in to your own user folder. (Note that it will be saved with the default settings rather than you modified settings. To save the modified effect, save a workspace. | |
This is a brief and specific guide to this topic, for more general information, see: MapInfo l - an easy guide for new users | |
Nigel
James
Bodleian Library 2001 |