The DEM must have some vertical datum to which elevations are referenced -- are the elevations in terms of the 1974 datum?

The MTL-NAVD88 parameter is set up such that if you looked at a gauge and there was a referenced MTL height and a referenced NAVD88 height, the value would be the difference of the two numbers referenced.

For example, at this station

MTL is 4.451 and NAVD88 is 3.134 meaning that MTL-NAVD88 would be approximately 1.3 meters.

NAVD88 can be substituted for whatever the datum of the elevation data set is. So you can convert your datum of elevation to MTL (often ~MSL) and then set the MTL-NAVD88 parameter to zero.

In the case of your figure, if your elevation data are in 1974 datum, MTL-NAVD88 would be 1.46 meters.

Been traveling, sorry about the delay in response -- hope this helps.

The MTL-NAVD88 parameter is set up such that if you looked at a gauge and there was a referenced MTL height and a referenced NAVD88 height, the value would be the difference of the two numbers referenced.

For example, at this station

MTL is 4.451 and NAVD88 is 3.134 meaning that MTL-NAVD88 would be approximately 1.3 meters.

NAVD88 can be substituted for whatever the datum of the elevation data set is. So you can convert your datum of elevation to MTL (often ~MSL) and then set the MTL-NAVD88 parameter to zero.

In the case of your figure, if your elevation data are in 1974 datum, MTL-NAVD88 would be 1.46 meters.

Been traveling, sorry about the delay in response -- hope this helps.