Decometer was the father of Loran, used by english troops during WW2.
It is a navigation instrument based on the same principle of the Loran-C.
There are approximately 393.7 inches in a decameter.
There is no such measure as a decometre. Unfortunately, there is a decimetre as well as a decametre. This makes it impossible to guess which one you could have meant and so the question cannot be answered.