The measuring line is the boundary line of the city, as Good News Translation makes clear.

The hill Gareb and Goah are of uncertain location. But since verse 38 marks out the northern boundary of the city, and verse 40 evidently gives the southern and eastern boundaries, it is assumed that the western boundary of the city is marked out by these two locations. If this is the case, then a person would move in a southwesterly direction from Gareb to Goah.

Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on Jeremiah. (UBS Helps for Translators). New York: UBS, 2003. For this and other handbooks for translators see here .