Range Report JBM and altitude density question...

[whoizrob]

I'm making up a new range card on JBM to use with Altitude Density readings and wasn't sure about what to set for the "zero altitude" value.

For "zero altitude" do I go off of my real altitude (638 ft) according to GPS, or do I use the Altitude Density for that day (2,198 ft)? FAQ wasn't too much help =P Thanks!

Rob

 

[sobrbiker883]

Re: JBM and altitude density question...

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: whoizrob</div><div class="ubbcode-body">I'm making up a new range card on JBM to use with Altitude Density readings and wasn't sure about what to set for the "zero altitude" value.

For "zero altitude" do I go off of my real altitude (638 ft) according to GPS, or do I use the Altitude Density for that day (2,198 ft)? FAQ wasn't too much help =P Thanks!

Rob
</div></div>

I use STP (standard temp/pressure) for variables other than altitude, and put DA in altitude. Lindy has a great writeup on using JBM with DA in the FAQ's somewhere here....

 

[PaulStafford]

Re: JBM and altitude density question...

FYI, GPS "altitude" is very inaccurate unless you have a WAAS-enabled GPS receiver (ie aircraft accuracy). This is the inescapable result of geometry (your 2D location is accurate because you have satellites all around your azimuth. Your altitude is being determined only by satellites above you, since your receiver can't see satellites through the earth

 

[sandwarrior]

Re: JBM and altitude density question...

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: whoizrob</div><div class="ubbcode-body">I'm making up a new range card on JBM to use with Altitude Density readings and wasn't sure about what to set for the "zero altitude" value.

For "zero altitude" do I go off of my real altitude (638 ft) according to GPS, or do I use the Altitude Density for that day (2,198 ft)? FAQ wasn't too much help =P Thanks!

Rob
</div></div>

For JBM, I use actual pressure (absolute off of your Atmospheric data tool or closest weather station) and true elevation and select "pressure corrected for altitude" OFF (the box is not checked).

 

[sobrbiker883]

Re: JBM and altitude density question...

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: whoizrob</div><div class="ubbcode-body">I'm making up a new range card on JBM to use with Altitude Density readings and wasn't sure about what to set for the "zero altitude" value.

For "zero altitude" do I go off of my real altitude (638 ft) according to GPS, or do I use the Altitude Density for that day (2,198 ft)? FAQ wasn't too much help =P Thanks!

Rob
</div></div>

Use the DA you zeroed at.
I start by making sure I have a table from JBM that matches my field data I actually shoot (usually 100, 200, 300, 500) with recorded conditions then I make cards for every 1500' DA from -1500 to 15000. I took a set of cards made with data that way with a rifle and cartridge I had only shot at those distances to Oklahoma a couple weekends ago and my elevation was spot on (within what i am capable of holding

) at a 6000' higher DA than my home range out to 1135 yds.
Now if it could help me with wind....

 

[whoizrob]

Re: JBM and altitude density question...

Using the DA for the zeroing conditions makes sense to me but I just wasn't sure about how to set up JBM to "know" that. The reply before yours hinted that perhaps using true altitude and unchecking the correction box would yield similar results?

Rob

 

[sobrbiker883]

Re: JBM and altitude density question...

Yes. With ballistics programs you can use temp, altitude, RH and corrected baro presure OR set altitude to "0" and use station pressure, RH and temp, OR you can use DA.

I run Bullet Flight with station pressure and JBM FTE with DA on my phone. I use DA with JBM online for making dope cards.

Once corrected to match actual field data they all jive within a tenth or two at distance.

 

[ctressler]

Re: JBM and altitude density question...

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sobrbiker883</div><div class="ubbcode-body">Yes. With ballistics programs you can use temp, altitude, RH and corrected baro presure OR set altitude to "0" and use station pressure, RH and temp, OR you can use DA.

I run Bullet Flight with station pressure and JBM FTE with DA on my phone. I use DA with JBM online for making dope cards.

Once corrected to match actual field data they all jive within a tenth or two at distance. </div></div>

This is the way to do it. If you have a Kestrel it's pretty simple to get the DA.