Shifting Swamp elevation

[moorer98]

So I have made my run and I noticed what appears to be a significant shift in my Swamp coverage near the edges of the swamp area (where the elevation is dipping) the Model shifted into transitional Salt marsh between the T₀ and the model start year. It is unclear to me how I shift the model so swamp can occur at a lower elevation.  Is that done in the elevation analysis window?  I was looking at that window with the 5 and 95 percentile information and was not completely sure how to interpret what I was looking at. Also I may have to shift a couple other wetland types. I feel like this should be an easy fix but I am not sure where to start.

Thank you for any assistance you can provide.

Rich

[marco.propato]

Hi Rich,
initially, SLAMM simulates the "time zero" step, in which the consistency of model assumptions is validated with respect to available wetland coverage information, elevation data and tidal frames. Due to simplifications within the SLAMM conceptual model, DEM and wetland layer uncertainty, or other local factors, some cells may fall below their lowest allowable elevation category and would be immediately converted by the model to a different land cover category (e.g. an area categorized in the wetland layer as swamp where water has a tidal regime according to its elevation and tidal information will be converted to a tidal marsh). These cells represent outliers on the distribution of elevations for a given land-cover type. Generally, a threshold tolerance of up to 5% change is allowed for in major land cover categories in SLAMM analyses (e.g. we allow the conversion to happen if it is not more than 5%)

So you can try to change the min/max allowable elevations in the elevation analysis window. However, this shift may be due to other reasons, for example the available elevations in those areas are very low and the tides data show that water there is tidal .... therefore a swamp can't be there for the model. So maybe is the tidal information that needs to be re-evaluated or the elevations around are too low and allow water to enter ....

Hope this helps,
Marco

[moorer98]

Thank you Marco,

That actually helps a lot!!

I will see if the conversion was over 5 percent and check the tide info too. 

I do have a follow up tide gage question.  In the NOAA tide gage database there are some gages that are no long active but still have the old readings, however, they did not have NAVD88 reading like the active gages.  Is there an alternate way to find a NAVD88 figure so one can use this old tide gage data or does it become invalid once NOAA stops recording data?

Thanks again,

Rich

[marco.propato]

For what concerns the tidal information we usually use it because it is a verified datum and it is definitively better than guessing.

To derive the elevation correction you can try to use the VDATUM software from NOAA at http://vdatum.noaa.gov/. It normally covers only coastal areas but it maybe sufficient for your application

Marco