According to http://www.odts.de/southptr/needham/measures.htm one Tang chi is 250mm, or about 9.8 modern inches. So an 18-"foot" Tang lance would be 4.5 metres, or about 14 feet 9 inches (assuming I've got my sums right....).

That would be more reasonable.

18 feet seems too long. the regulations would be very interesting.

That definitely sounds too long.

Q: Was the spear head during the ancient times specialized diamond-shaped?

As to the precise length of the spear of the Han dynasty, definite information was obtained from Tomb No. 94 at Shaogou, Luoyang. The tomb dates to the first half of the third period which is equivalent to the late period of the Western Han dynasty. The long spear was found lying on the left side of a skeleton. The iron spearhead has an overall length of 47 cm, and the edge of blade is 20 cm long. Below the blade is the hollow cylindrical handle which has an inner radius of 2 cm. The blade was covered in rotten wood, which may have been the sheath. The wooden shaft was inserted in the hollow part of the cylindrical iron handle. The other end of the shaft was capped with a bronze ferrule, called dun. The wooden shaft does not remain, but the bronze dun still lies in its original position. The distance between the tip of the iron spearhead and this bronze ferrule is 250 cm. This should be the original length of the spear.