Topic: Elevation/Altimeter Settings

[m.c.crockett]

 have been interested in meteorology since learning that my mother was a meteorologist when she was a WAF assigned to SHAEF during World War II.  This Christmas, I received a Vantage Pro2 as a gift from the family.  I ordered the WeatherLink hardware and software on 31 December 2007 and am waiting for it to arrive.

Some of the information needed to configure the console has been relatively easy to obtain, i.e. the LAT/LONG position of my house.  The elevation has been a bit more problematical.  My lot is on the side of a hill with a 40-50 foot elevation difference between the southwest and northeast corners of the lot.

I'm using a range because the topographical map tools show my lot at different elevations because they align the street maps to the USGS topological maps differently.  Another problem is that the USGS maps were created before the homes were built and its difficult to determine how the "cut and fill" changed the elevation.

How did you determine the elevation of your Vantage Pro2 site?

A second question is whether the elevation used is the ground level at the site or the base of the Vantage Pro2 ISS?

This raises a third question related to participation in CWOP.  Is the altimeter setting just the computed standard pressure based on elevation?

[racenet]

How did you determine the elevation of your Vantage Pro2 site?

Just looked at an online USGS map, overlayed with a street map, found my house and took the elevation and added how high above that my console was sitting.

A second question is whether the elevation used is the ground level at the site or the base of the Vantage Pro2 ISS?

It is determined by how high above that elevation the console is, not the ISS.

[m.c.crockett]

That's my dilemma.  The topographical overlay to the street maps isn't consistent.  There seems to be a 30 foot difference.  The City of Thousand Oaks has an ArcGIS server maintained by the survey department.  It seems to be pretty accurate but uses old USGS topographical map overlays that do not include modifications to the topography made be the developer of my tract.

[bcrawfo2]

Download and install Google Earth.
Wherever your mouse is on a map, it will show the elevation.
Then figure out how much above the natural elevation your console is.   Take into account basement, crawlspace, etc.

[George Richardson]

FWIW

It doesn't matter! Pick a number and swear by it. I was fortunate that I KNOW what my elevation is but barometer readings gave me fits for months. When you get VPLive from SoftWx, you will be reported 6 different barometer pressures. The three main ones (I think) are Station pressure, Barometer (Sea Level), and altimeter which airports and CWOP use.

Get, read and heed the CWOP "Weather Station Siting, Performance, and Data Quality GUIDE" and remember that unless you rent space at an airport, you aren't going to be able to follow all Siting instructions.

Actually, IMHO, Siting the rain bucket is the most beneficial to best location of all your weather instruments.

Best of Luck and . . . . . Have FUN!!

George

[Dal]

Download and install Google Earth.
Wherever your mouse is on a map, it will show the elevation.
Then figure out how much above the natural elevation your console is.   Take into account basement, crawlspace, etc.

What happens if you're on the side of a hill with the bottom part of the section lower than the rest and Google Earth showing only one elevation for the whole section? Or am I being too pedantic here?

[Anthony]

I agree elevation is critical but not that critical. If the elevation is off by 50', 100' or more that one thing. But if you only off by a few feet I wouldn't worry about it.

How did I determine elevation? I used a GPS at the location of the sensors not the console.

[wmiler]

Once you have a CWOP id assigned, you can check on their site for elevation reasonableness by going to http://weather.gladstonefamily.net/site/(your CWOP id) and moving the marker on the map. When you release
the marker, the text on the page will change and give you a best guess elevation.

Mind you, this is the ground elevation, not the elevation of your barometer. If you have your barometer (console)
on the second story of your home, add about 20 feet to your elevation.

[Anole]

I agree elevation is critical but not that critical. If the elevation is off by 50', 100' or more that one thing. But if you only off by a few feet I wouldn't worry about it.

How did I determine elevation? I used a GPS at the location of the sensors not the console.

You have to be careful using a GPS for elevation. A good explanation of why is here:
http://www.ja-gps.com.au/whatisgps.html

Scroll down the to section on elevation.

[m.c.crockett]

I agree elevation is critical but not that critical. If the elevation is off by 50', 100' or more that one thing. But if you only off by a few feet I wouldn't worry about it.

How did I determine elevation? I used a GPS at the location of the sensors not the console.

On a parcel with a 40-50 foot grade difference, doesn't it make a difference?  Isn't barometric pressure based on the pressure needed to raise a column of mercury/water 32.2 feet?

[m.c.crockett]

Once you have a CWOP id assigned, you can check on their site for elevation reasonableness by going to http://weather.gladstonefamily.net/site/(your CWOP id) and moving the marker on the map. When you release
the marker, the text on the page will change and give you a best guess elevation.

How long does it, normally, take to have a CWOP ID assigned?  I requested one on 30 December 2007 after returning from holiday.

[wmiler]

How long does it, normally, take to have a CWOP ID assigned?  I requested one on 30 December 2007 after returning from holiday.

Give or take, about a week although this is holiday time, I'm sure with all the Santa delivered weather stations CWOP may be a bit backlogged.

[George Richardson]

"On a parcel with a 40-50 foot grade difference, doesn't it make a difference?" Why don't you plug the lower elevation into your base settings, read the Barometric pressure (which is calculated to sea level pressure) then reset your elevation setting to the higher possible elevation and see what the new calculated sea level pressure is. My station has a 0.034inHG calibration and I know what my elevation is. The Davis instruments are very good but they can be, and in some instances, need to be , calibrated. Most of the contributors to this site have a manual rain gage to confirm or give a point of reference to the tipping bucket which have been accused of being as much as 20% off from the factory. Usually the manual gage shows the Davis tipping bucket to be much closer to accurate than that but still in need of tweaking. My CWOP ID is CW6097

[tinplate]

Technically, there are several definitions of elevation used by the NWS for stations at airports.. The airport elevation is the highest point of the runway. The station elevation is the official reference point for the data. The sensor elevation is the elevation of the actual barometer instrument. For a home weather station, you could use the elevation where your ISS is, or where your console is. I chose the ISS elevation. Unless the difference in elevation between the two is large (greater than 30-50 feet), it really doesn't matter which you pick as far the altimeter data is concerned. Once you've calibrated your barometer to a reliable reference, the altimeter you report will be the same whichever elevation you used. Just make sure the elevation you enter into the console is the same elevation you report to CWOP.

[roakey]

I probably have at least a 40’ difference on my land, but it was easy enough to pick out my exact site within my land on a topo map.  A topo map told me my site was at 6640, Google Earth gave me 6636, and when I stood at the exact location with my GPS (Garmin V) it bounced back and forth between 6636 and 6640.

Frankly I was amazed at the high degree of correlation.

If you live near an REI, they have a computer kiosk that can print out topo maps.  Zoom in on your property as much as possible and pull the altitude off of the preview, no need to buy a map.

Roak

[m.c.crockett]

Thanks for the reminder about Google Earth displaying the elevation at the bottom of the window.  The elevation reported was fairly close to the elevation that I had guessed for the "cut and fill" needed for a level foundation for my home.

So, I have the ISS at an elevation of 251 meters and the console at 250 meters.  How do I convert this to an altimeter reading?

Point Mugu, Naval Air Warfare Center, has an elevation of 10 feet.  The altimeter settings change depending upon the current barometric pressure.

Is the altimeter setting used to adjust the current barometer reading relative to sea level?

[RioRanchoNMWX]

When all else fails get a 3D GPS...by taking several samplings you can get a pretty accurate reading.

[SpartanWX]

I agree elevation is critical but not that critical. If the elevation is off by 50', 100' or more that one thing. But if you only off by a few feet I wouldn't worry about it.

How did I determine elevation? I used a GPS at the location of the sensors not the console.

You have to be careful using a GPS for elevation. A good explanation of why is here:
http://www.ja-gps.com.au/whatisgps.html

Scroll down the to section on elevation.

Maybe you can "calibrate" or check the accuracy to some extent if you have a USGS benchmark nearby.  They are benchmarks with established elevations.  You can use it in a differential GPS fashion by taking a measurement at the benchmark and comparing the elevation at the benchmark to that on the GPS.  Apply the difference to the measurement at the location at the sensor... and return to the benchmark to make sure the bias has not drifted.  I've never tried it, but I am guessing it ought to get you within a few feet which is more accurate than the GPS can be by itself.

Or maybe not *shrug*

[neondesert]

Here is how the CWOP FAQ @ http://www.wxqa.com/faq.html recommends finding your lat./long. and elev.

4. How can I find my latitude, longitude and elevation? While there are several different ways, one is to go to ACME mapper and enter your nearby town and your state at the bottom and press "find". Then change to a high resolution image and double click on your actual location. Your latitude and longitude is then given in the "Lat/Long" box. Once you have your latitude and longitude in decimal degrees, copy and paste (without the quotes) "http://www.earthtools.org/height/40.0545/-105.20783" into your browser and then change the first number to your latitude and the second number to your longitude and then press enter. In the resulting text, you'll see your elevation in feet and in meters which is what is used in the CWOP database.

[MesquiteWx]

Very interesting that I came across this thread. I was wondering about this this morning. So do you need to compensate for the ft it is off the ground. I have my sensors on my roof, or actually hanging off the side. So do I need to add some ft to this? I was going to do it with my GPS but after seeing that was not a good idea I guess I could get out the trusty ol tape measure. I put my handheld GPS on the ground and the elevation would never stabilize so it wouldn't be a good option anyways.