How To: Create labels on contour lines using Mask and Halo
Follow these steps to place a contour label in ArcMap.
Right-click the layer name in the ArcMap table of contents and select Properties.
Click the Labels tab to set the contour line properties.
A. Verify the label features box is checked. B. In the Method dropdown box choose 'Label all the features the same way'. C. In the Label dropdown box choose the field that contains contour values. D. Click Placement Properties. E. Set the constraint placement to 'On the Line'. Make sure you uncheck the 'Above line' option on the left.
Click the Symbol button to view the Symbol Selector window and adjust the labels.
A. Select the desired label font, color, and size. B. Click the Properties button. C. Click the Mask Tab. D. Click the Halo button and adjust the Halo to the desired size.
Click OK to close the Editor window, and OK again to close the Symbol Selector window.
Click Apply > OK, and Zoom to view contour labeling.