Your map layer contains a multilayer line symbol whose widths may result in aliasing. This is due to the fact that you have defined a line width that is less than one pixel in width. The resulting map display (using aliasing) in a map service will differ slightly from your ArcMap display.
Solutions
- Do nothing. You can accept the display difference as is.
- Alternatively, automatically correct incompatible line widths on conversion by choosing the option on the warning's shortcut menu.
- Alternatively, you can change the width of the multilayer line symbols to better support the screen display resolution of most computers. Use a line width of one pixel or higher.
More information
Map services utilize a new graphics engine that is capable of rendering line features using the true noninteger coordinates of your vector data. In an ArcMap display, the coordinates are rounded to the nearest integer, resulting in some imprecision.
However, because the symbology in ArcMap was designed for best appearance using the ArcMap display engine, certain types of symbology may exhibit different behavior in a map service.
Learn more about drawing differences between the ArcGIS drawing engines
For most users, the Automatically Correct Line Widths option will result in rendering equivalent to ArcMap by rounding the line widths in the same way that the ArcMap display pipeline does.