This is a point not realized by most men who install map and tack systems, but it usually sooner or later sounds the death-knell of the tack system.

228 GRAPHIC METHODS

Generally the tacks are placed in the maps one by one as agencies are established or as data are obtained from correspondence. After the correspondence by which each tack was located has gone to the correspondence files, there is ordinarily no list showing the geographical location of the tacks. If a single tack is found loose in the bottom of

a drawer of a cabinet system, or on the floor of an office where there is a wall map, it causes distrust of the whole tack installation. When there is no list showing the geographical location of different

b

Fig. i86. A Contrast Between Long, tacks, the one tack which is

SrrFSou{'.InfGra"3,e"d <"" "' ?'='« -""»' b-= P"' b-'' Map Pins Made with Short Needle- without checking over correpointssothatthePinsMaybe Pushed gpondence and records which ui until the Heads Touch the Map . i i i r- -o

may extend back tor years, jbven

when a list of tack locations is at hand, the loose tack cannot be replaced without checking the location of all the tacks on the map one by one to determine by a process of elimination where the loose tack came from. In the ordinary course of human events it is not likely that a tack falling out of a map would be found to give warning that the map record is no longer accurate. The tack system using long projecting tacks may therefore contain unsuspected inaccuracies just because tacks may have come loose. The unpleasant suspicion that a map record may be inaccurate, because of the long tacks falling out, sometimes causes a man to abandon the tack system entirely, believing that it is not reliable enough to give data on which important decisions must be based.