For what country?
Has this not already been mapped before for your region?
Do you have an accurate coastline outline for your region?
- How was this coastline derived?
You mention it is being surveyed with XY locations of each LWM/HWM
- Would really need to get more info around this data collection before giving you a good answer.
A crude approach which is just me thinking off the top of my head.
I will try and put some more thought into this tomm when I have more time.
- first get this XY data into ArcGIS and get it plotted out.
- Get a DTM for the region. Ideally a DTM with good bathymetric heights as well.
- Using Spatial Analyst create an interpolation of the XYZ data (Z being the attribute of either the HWM or the LWM). Perhaps use the inner coastline polygon layer as a barrier (?) I would 1st sugggest doing an Inverse Distance Weighted interpolation, setting the max distance based on how far your points are apart.
- Using the grid calculator, subtract the DTM with the interpolated layer you created above.
- Remove any negative values from the result.
- This should give you a layer with the perimeter being the LWM/HWM depending on which results you interpolated above. You could then create a polygon extent of this grid and this would be your result.
I imagine this won't be the best answer (not in its current state) but perhaps it will cause others to suggest a better, more accurate answer. Or the author to give more info...
TBH, at the mo, I am reluctant to put more effort/time in until the question is phrased better.