Index Contour Definition: A Thicker Line for Easier Map Reading

When creating a topographic map, cartographers use various techniques to make it easier to read and understand. One such technique is the use of index contours, which involve accentuating contour lines with a heavier line weight to distinguish them from intermediate contour lines. In this article, we will explore what index contours are, how they differ from regular contours, and provide step-by-step instructions on how to create them using ArcGIS software.

What Are Index Contours?

Index contours are thicker contour lines that are used to mark important elevation changes on a map. These lines are typically drawn at regular intervals, with every fourth or fifth contour line shown with a thicker line. The purpose of index contours is to provide a visual cue for the viewer, making it easier to read and understand the topography of an area.

How Are Index Contours Different from Regular Contours?

Regular contours are lines that connect points of equal elevation on a map. They are used to show the shape and relief of an area, with closely spaced contour lines indicating steep slopes and widely spaced lines indicating gentle slopes. Index contours, on the other hand, are thicker lines that are used to mark important elevation changes.

Creating Index Contours in ArcGIS

To create index contours in ArcGIS, you can use the following steps:

1. Open your contour feature class attribute table and add an integer field that stores ones for index contours and zeros for all others.

2. Use the Calculator tool to create a new field that distinguishes between index contours and regular contours. In this example, we will use VBScript code to achieve this.

3. Copy the following code into the Pre-logic VBA Script Code display window:

```

IndexInterval = 50 ' set to interval of index contours

Cont = ' Set to contour height field

If ( (dCont / dindexInterval) - i = 0) then

' symbolize the layer using a categorical classification based on the new field

Type the letter j in the text input field at the bottom of the dialog box and click OK.

```

4. Save your changes and re-symbolize your contour feature class using the new field.

Benefits of Index Contours

The use of index contours has several benefits, including:

* Improved readability: Index contours make it easier to read and understand topographic maps, especially when the contour lines are numerous and closely spaced.

* Enhanced visual appeal: The use of thicker lines for index contours can make a map more appealing visually, making it easier to distinguish between different elevation changes.

In conclusion, index contours are an important tool in cartography that can be used to make topographic maps easier to read and understand. By following the steps outlined above, you can create index contours using ArcGIS software. Whether you're a professional cartographer or just starting out, understanding how to create index contours is an essential skill for anyone working with geographic information systems (GIS).

Index contour definition