Since aeccland.shx is a compiled file, you cannot easily edit it directly. However, the "interesting" aspect of this technology is that you can build your own version to standardize your company's drawings.

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Are you experiencing a when opening a specific drawing, or are you looking to customize a linetype using these shapes? ERROR WITH LINETYPE FILE: acad.lin - Forums, Autodesk

If you are an AutoCAD or Civil 3D user, you have likely encountered the "Missing SHX Files" dialog box specifically requesting aeccland.shx