Linear Referencing on the NHD Linear referencing on the NHD is interesting because even though there are three route systems only one of the systems is linearly addressed. The drain and landmark routes, which have the polyline M geometry, have an M-value randomly assigned to them by ArcInfo. The measures on route.reach are assigned in an order that opposes the direction of the coordinates and are applied following the Relative Addressing scheme. There are three types of reaches, which each of their own methods of assigning the measure. Linear reaches are located in streams or a waterbody with only one in and out flow.
Discontinuous reaches occur in swamps or marshes.
Linear reaches have a relative measure assigned to them that starts with a value of 0 at the downstream end of the reach and extends to 100 at the upstream end of the reach. Discontinuous reaches, to distinguish them from linear reaches, have a measure from 100 to 200 applied in the same fashion as for linear reaches. Branched reaches use a methodology that combines the addressing used in linear and discontinuous branches. If there is a distinguishable main path, it is assigned values from 0 to 100 as for a linear reach. The branches are then given measures like those used for discontinuous reaches from 100 to 200.