Elevation Profile question

[hedbonker 5]

In the elevation profile pane there is a red line that indicates a minimum altitude.  I have a plan that goes from KDAY in Dayton Oh. to KERI in Erie Pa.  The elevation profile indicates a minimum altitude of 2500 feet yet when I have the show minimum altitude option turned on for the map I can see that these indicate altitudes above that for each quadrant that the plan passes over.  2900 feet is actually the lowest indicated on the map.  When I look at a current sectional, those number do not jive with LNM's.  The highest elevation on the sectional is 2200 feet around Cleveland.  What am I not understanding here?

Thanks in advance!

[Gason71 128]

IMHO they are  differences between sectional chart and profile data taken from online/offline elevation grids.
LNM uses world global NOAA elevation data  that you can download from here: https://www.ngdc.noaa.gov/mgg/topo/gltiles.html
If you are interested in, here is explanation/report how NOAA global elevation data were calculated: https://www.ngdc.noaa.gov/mgg/topo/report/globedocumentationmanual.pdf

Regards,
Piotr

[hedbonker 5]

Thanks so much Piotr.  Makes sense as to the differences on the map verses the sectional.  I would still like to know about the red line tho.  Why it is set to such a low altitude.

Thanks in advance!

[Gason71 128]

1 hour ago, hedbonker said:

 I would still like to know about the red line tho.  Why it is set to such a low altitude.

 

Quote from LNM manual:  "Orange lines show minimum safe altitude for flight plan segments. Red line shows overall minimum safe altitude"

As I've reviewed direct route KDAY->KERI it seems the red line is calculated arbitrary as 1000 ft minimum altitude rounded up to closest 500ft over highest route obstacle.
It gives red line at 2500ft as is showed in my LNM (I use offline downloaded grid data - because LNM works faster 🙂 ), the highest enroute obstacle I've found on vertical profile is 1368ft.
On sectional VFR I've checked the same point/peak is showed as 1629ft obstacle (between LEKNE/DANEI IFR fixes) near W. Liberty and Urbana.

This red line gives you minimum altitude where you haven't to change altitudes because of obstacles  😎

If you create longer route with many segments - then for every segment additionally should be displayed orange lines .
Orange segment lines can be below red line (or not exist if red and orange are at the same level), but never vice versa.

Regards,
Piotr

 

[edit]
ps. To be clear - remember that minimum safe altitude from LNM is not the same what MSA from IFR sectionals!!!
MSA, MSE, MRA, (n general all avove MOCA) ... etc are altitudes from IFR charts/procedures.
On the other hand red line in LNM is for orientation only, you know that below this line you can encounter enroute obstacles 🙂
 

[edit2]
I've found at LNM/Tools/Options the option where value of  Minimum altittude buffer can be typed in 🙂
Default is 1000ft but you can change this to your own value, for example over mountains OCH should be 2000ft minimum, over congested area/water 1000fr, over other terrain 500ft - of course this is about VFR flights. In case of IFR flight the defined altitudes (on IFR chart) - MSA, MSE, MRA, ... are taken into consideration or 2000ft (mountain)/1000ft (other) rule when MOCA/MSA/MSE/MRA are not defined. I've written about FAA rules, ICAO rules can be slightly different.
 

Edited March 30, 2021 by ppgas

[hedbonker 5]

Thanks so much Piotr.  It all makes sense now!

Thanks in advance!

[albar965 1,047]

Red line is calculated from the actual elevation (the green stuff on the profile) below the flight plan legs and not the MSA or MORA numbers.

This was introduced when MORA was not available yet.

Alex

[hedbonker 5]

Thanks Alex!

[Gason71 128]

2 hours ago, albar965 said:

Red line is calculated from the actual elevation (the green stuff on the profile) below the flight plan legs and not the MSA or MORA numbers.

This was introduced when MORA was not available yet.

IMHO this solution that exists now is better (more "blades" and possibilities in this Swiss knife tool 🙂 ). MORA/MSA/etc are required for IFR flights.
Now it's possible to set the value that is needed for VFR flights (for example 500ft if terrain is empty and flat) and it works fine, when IFR flight is performed it's possible to set 1000ft or 2000ft.
And of course always is possibility switching on MORA grid and manually  checking calculated altitude with MORA values.
 

Regards,
Piotr