Converting Elevation Analysis from HTU to NAVD 88 meters

[jmkassak]

Hello,

I am interested in converting the results of the elevation analysis to true elevation in order to allow for easier consideration by local experts of the elevation ranges in which the various habitat types exist.  To do so, I would think that the following conversion equation would be appropriate:

[results in HTU] * [HTU in meters] = Result in meters adjusted to MTL = 0
[results in meters adjusted to MTL = 0] - [NAVD 88 correction factor] = Results in meters, NAVD 88

So for example:
the 5th percentile elevation for Transitional Salt Marsh is -1.54 HTU. 
HTU in this system is 0.11 meters.
the 5th percentile elevation in meters is thus -1.54*0.11 = -0.17 meters (relative to a system in which MTL = 0)
The NAVD 88 correction factor in this area is 0.205 m
-0.17 meters - 0.205 m = -0.37 m NAVD 88

So -0.37 m NAVD 88 is the 5th percentile of the elevation range for Transitional Salt Marsh.

I would be grateful is someone could check my math on this and let me know if this is the appropriate way to do this conversion.  I thought it was, but got confused about whether the results of the analysis were already in NAVD88.  This came up because I noticed that in the Conceptual model output it calculated the "Min in HTU" and "Max in HTU" values based on the salt elevation in NAVD 88, rather than on the salt elevation corrected such that MTL = 0.

Thank you!

[Amy Polaczyk]

Hello,

The calculation you need is actually much simpler. Since the vertical datum for both HTU and "meters" is MTL, multiplying the elevation in HTU by the definition of HTU in meters is sufficient to determine the elevation in meters. What you have done below should do the trick:

the 5th percentile elevation for Transitional Salt Marsh is -1.54 HTU. 
HTU in this system is 0.11 meters.
the 5th percentile elevation in meters is thus -1.54*0.11 = -0.17 meters

You do not need to include the NAVD 88 correction in the calculation (unless you specifically require the elevation data relative specifically to NAVD).

It is important to keep in mind that this conversion may change over the study area if the tide range is variable.

The good news is that in SLAMM 6.1 (to be available at the end of the month) the user has the option of viewing the elevation analysis in both HTU and meters.

You also wrote:

This came up because I noticed that in the Conceptual model output it calculated the "Min in HTU" and "Max in HTU" values based on the salt elevation in NAVD 88, rather than on the salt elevation corrected such that MTL = 0.

This shouldn't be the case - could you describe how you determined that?

Best regards,
Amy

[jmkassak]

Hi Amy,

Thanks very much for your response.  Regarding the elevation conversion, I did need to see it in meters NAVD 88.  This analysis was run just for one subsite for which we input one correction factor, so the variability in the tides and correction factor should not be an issue.

As far as the salt elevation conversion to HTU.  I was looking at the first section of the elevation analysis.  It shows the min elevation for several habitat types as 1 Salt Elevation.  It then also shows the minimum in HTU, in this case 1.09091.  To convert from 1 salt elevation to HTU I assume you would divide the salt elevation by the HTU in meters value.  So in this case, where I have a NAVD salt elevation of 0.12 (which is what is entered into the model) and an HTU of 0.11 m I end up with a salt elevation in HTU of 1.09091. 

If, instead, the salt elevation were based on salt elevation where MTL = 0, I end up with a different number.  In that case, I would convert my NAVD salt elevation of 0.12 m using the correction factor of 0.205 m to get 0.325 m.  If I then divide that by the HTU in m (0.11 m) I get 2.9455 m for the salt elevation in HTU.

Please let me know if I'm interpreting something incorrectly here.  This site is quite a challenge in terms of fitting it to the conceptual model and the salt elevation appears to be one of the most critical issues.....

Thanks!
Jen

[Amy Polaczyk]

Hi Jen,

I have an idea as to where the confusion might be. You said:

...where I have a NAVD salt elevation of 0.12 (which is what is entered into the model)...

In the Site Parameters, you should be inputting your salt elevation in meters above MTL, not NAVD. Since the model is expecting a value relative to MTL for that parameter, you may be applying the NAVD88-MTL correction twice...

If you'd like you can send project SLAMM6 file to us and we can have a look at it.

[jmkassak]

Hi again,

Well that would certainly make a difference!  I don't see anywhere in the Tech Documentation or user manual that specifies that salt elevation should be in MTL=0 terms, but since the correction factor is specified I assumed it wanted all input information (LiDAR, salt elevation) in NAVD 88.  So we are going to have to re-run the model and elevation analysis with that revised parameter.  We are also making a few revisions to our wetland categorizations (we are not using NWI data) and to several of the other parameters.  We anticipate re-running the model early next week, and it would be great if you could take a look at the file after that.  I'm having quite a bit of difficulty with the Elevation Conceptual Model being very far off what we are seeing with our data (noticeable both from the elevation analysis and extensive conversion that happens between the initial condition and T=0 (see my other posts on these topics)).  It would be great if you were able to take a look at our project file to see if I'm making an error somewhere.

Please let me know where I can send this information.

Thanks!
Jen

[Amy Polaczyk]

Jen,

You can send the file to me at: apolaczyk@warrenpinnacle.com

Have a nice weekend!
Amy

[jmkassak]

Thanks Amy.  I sent everything over to you on 1/14.  Would you mind just confirming that it got to you?

Thanks!
Jen