Author Topic: Help with Barometer (Read 4940 times)

KenD · « **on:** January 30, 2010, 10:09:31 AM »

My new station has been up and running for about a week now. My initial report from MADIS gave me two thumbs up across the board but today I noticed I only had one thumbs up for my barometric pressure readings. I've noticed that I seem to be running high in my barometric readings compared to adjacent stations by a few hundreths of an inch, generally .01 to .06. I'm wondering if I need to calibrate the barometer or take another look at my elevation. I've verified my elevation using a Garmin hiking GPS. I don't know my elevation above sea level or the correct barometric pressure for my elevation so I've had the GPS default to the GPS altitude. I've taken several readings outside and settled on 970 feet as being my mean elevation across the various readings.

MADIS reports my elevation as:
Elevation: 295 meters (969 feet) -- validated against 273 meters (894 feet) from NED 1/3rd arc-second: Eastern United States

If I'm off on the elevation by, say, 50 feet, would that cause the error reported by MADIS: http://weather.gladstonefamily.net/site/D4380

970 feet feels about right based on my elevation compared to that reported by adjacent stations - there's a few low hills around here so there's probably a 30 - 50 foot change in elevation as you drive around my area. I seem to be 70 to 40 feet above the surrounding stations. Should I consider re-calibrating my barometer to correct for the error reported by MADIS? Or try to verify my elevation using another method (assuming that makes a difference)? Or just trust the readings?

George Richardson · « **Reply #1 on:** January 30, 2010, 10:43:05 AM »

Hey Ken,

How are you sending data to CWOP. Some software sends the "wrong" stuff. IMHO, VPLive is the best program for submitting data to CWOP.

"Should I consider re-calibrating my barometer to correct for the error reported by MADIS?" Yes but not yet.

"If I'm off on the elevation by, say, 50 feet, would that cause the error reported by MADIS:" Maybe.

"Or try to verify my elevation using another method (assuming that makes a difference)? Or just trust the readings?" Yes - Google Earth or local Topographic maps. Probably shouldn't do anything major until you get a big summer high to tweak to. In the end, you may need to add a small offset to your Davis console.

JMO

George

W3DRM · « **Reply #2 on:** January 30, 2010, 11:51:52 AM »

Ken,

I will echo what George said about using VPLive. It will save you lots of headaches trying to "chase" your barometer readings.

Also, occassionally I get swings in my CWOP quality reports (both barometer and temp) that would seem to tell me I am way off the mark. HOWEVER, DO NOT MAKE ANY CHANGES. Just sit back and let a few days go by to see if your readings come back into range again. Remember the MADIS data is taken from areas surrounding you and they may contain data that is very different than yours due to microclimate effects.

As to setting of your altitude, that is critical so it needs to be as close to perfect as possible. Google Earth can be good but you really need to find some local topographic maps of your area. They are available on-line as well as at your local sporting stores. Use a GPS to find your exact lat/lon and go from there. Don't forget to add any height your console is above the outside ground level as shown in the topo maps. I would not put in any offsets to my weather station until you get everything else set correctly. Then, watch your reports and make adjustments as necessary.

SlowModem · « **Reply #3 on:** January 30, 2010, 12:29:06 PM »

Quote from: W3DRM on January 30, 2010, 11:51:52 AM

As to setting of your altitude, that is critical so it needs to be as close to perfect as possible. Google Earth can be good but you really need to find some local topographic maps of your area. They are available on-line as well as at your local sporting stores. Use a GPS to find your exact lat/lon and go from there. Don't forget to add any height your console is above the outside ground level as shown in the topo maps. I would not put in any offsets to my weather station until you get everything else set correctly. Then, watch your reports and make adjustments as necessary.

I have found that http://terraserver-usa.com/ is a good source for topo maps. they have some interesting aerial pictures, too.

KenD · « **Reply #4 on:** January 30, 2010, 01:26:27 PM »

Quote from: George Richardson on January 30, 2010, 10:43:05 AM

Hey Ken,

How are you sending data to CWOP. Some software sends the "wrong" stuff. IMHO, VPLive is the best program for submitting data to CWOP.

I'm using Weatherlink for now.

dalecoy · « **Reply #5 on:** January 30, 2010, 01:48:52 PM »

Just checking - you are setting the elevation of the console?

Weather Display · « **Reply #6 on:** January 30, 2010, 02:14:51 PM »

WD also sends the correct barometer reading to CWOP (option to use the send altimeter calculated barometer reading)

KenD · « **Reply #7 on:** January 30, 2010, 03:51:23 PM »

Quote from: dalecoy on January 30, 2010, 01:48:52 PM

Just checking - you are setting the elevation of the console?

Yes, and after studying a local topo map from trails.com and taking more readings with my GPS, I've decided to change my elevation from 970 to 920 feet then watch it for a few days. I think that's about as close as I'm going to get without a commercial-grade GPS.

KenD · « **Reply #8 on:** January 30, 2010, 04:54:55 PM »

Looks like that gets me within .01 or .02" of the stations closest to me.

Wx4U · « **Reply #9 on:** January 30, 2010, 10:25:33 PM »

Ken, I looked up KGALILBU8 and MAP388 using the Lat/Lon provided on Weather Underground. You are just a little over 1000 ft apart and the elevations at those location are different by about 65 ft ( your location being higher). Remember your barometer is located in the console, so you will have to add the height of where it is physically located in the house above the outside elevation. 10 ft of elevation equal .011 inches of pressure. Therefore, your barometer should be about .065 above that of the MADIS MAP388.
I assume by your lat/lon you are located between the circle at the wsw end of Mayflower Ct and the circle at the wsw end of Colonial Trail.
Don't worry to much about it, it will settle and you will be able to find more exact methods of fine tuning location and elevation.
Bob

KenD · « **Reply #10 on:** January 31, 2010, 11:15:18 AM »

Thanks. I'm on a slope so it's a bit of guess work using topo maps. I think 920 feet gets me within 5 feet based on the range of elevations shown on the various maps I've looked at and my GPS readings which seem to have a margin of error of around 20 feet.

dalecoy · « **Reply #11 on:** January 31, 2010, 12:45:20 PM »

Quote from: KenD on January 31, 2010, 11:15:18 AM

Thanks. I'm on a slope so it's a bit of guess work using topo maps. I think 920 feet gets me within 5 feet based on the range of elevations shown on the various maps I've looked at and my GPS readings which seem to have a margin of error of around 20 feet.

I don't know how accurate your GPS is, for horizontal measurements. But here's an interesting quote:

"Vertical (elevation) data from GPS units are generally less accurate than horizontal data. In fact, a GPS receiver with a 3-ft. rms horizontal accuracy may only have 10-15 ft. rms elevation accuracy. "

Wx4U · « **Reply #12 on:** January 31, 2010, 01:37:13 PM »

In this area Google Earth is more accurate than a normal GPS. I checked against the professional survey that recently surveyed my property for natural gas royalties and it was within a foot. Last night as I looked over your area I did notice that the resolution here is better than there.
I think you are okay.

db4 · « **Reply #13 on:** January 31, 2010, 06:55:56 PM »

Hello All, I believe we have a resource that will help with barometer accuracy checks. we have updated our barometer calibration page with an interpolator of nearby stations. It can be seen at www.starpath.com/barometers we also tap into the National elevation data set for an accurate elevation. must have a correspondingly accurate location, especially if on a slope. Both resources (pressure and elevation) work worldwide. glad to answer questions here about this or at helpdesk@starpath.com comments and suggestions would be much appreciated. --david burch

George Richardson · « **Reply #14 on:** January 31, 2010, 07:28:34 PM »

Hey David,
Your elevation thingie worked pretty well. The barometric pressure didn't.
George

db4 · « **Reply #15 on:** January 31, 2010, 07:33:53 PM »

can you tell me the lat lon you used and i will check it

KenD · « **Reply #16 on:** February 01, 2010, 09:44:41 AM »

Latitude: 33° 51' 41" N (deg min sec), 33.8613° (decimal), 3351.68N (LORAN)
Longitude: 84° 5' 21" W (deg min sec), -84.0892° (decimal), 08405.35W (LORAN)

Thanks. I checked out your web site and it reports 894' but I think that's at the bottom of the slope on which my house sits. The top of the slope shows around 938 based on topo maps so I've settled on 920' which is now getting me two thumbs up from MADIS.

KenD · « **Reply #17 on:** September 04, 2012, 08:42:36 PM »

Huge bump with this one.

So I've been running without a hiccup for over two years with thumbs up across the board from MADIS till the pc running my weather software died. Replaced it and reinstalled WuHu. Everything is communicating and I'm getting two thumbs up from MADIS on everything except... the barometer, again. I checked my console to make sure it still had my correct elevation, and it did. I've gone through the WuHu settings several times to make sure I didn't miss an elevation setting and can't find anything. I've checked surrounding stations on Weather Underground and I seem to be right in the range with everyone else.

But MADIS gives me an L3 spatial consistency error and a thumbs down:
http://weather.gladstonefamily.net/site/D4380

I should add that I was down for about 3 weeks. Any ideas? Thanks!

C5250 · « **Reply #18 on:** September 04, 2012, 11:04:56 PM »

Being that your elevation is less than 1000', I would suggest that you set your elevation to 0' and apply an offset. This assumes that you are not using a program that corrects the Davis generated sea level reduced pressure to altimeter. The latter being what CWOP expects. Keep in mind that your pressure is measure by the console, so you have to correct to the altitude of that.

KenD · « **Reply #19 on:** September 05, 2012, 07:31:41 PM »

Quote from: C5250 on September 04, 2012, 11:04:56 PM

Being that your elevation is less than 1000', I would suggest that you set your elevation to 0' and apply an offset. This assumes that you are not using a program that corrects the Davis generated sea level reduced pressure to altimeter. The latter being what CWOP expects. Keep in mind that your pressure is measure by the console, so you have to correct to the altitude of that.

Well, everything was working perfectly with my console elevation set to 920'. The only thing that changed was replacing the PC and reinstalling Wuhu. Is there something I missed with respect to CWOP? It's been so long since I originally set everything up that I'm wondering if there is an elevation setting either in my software or in my station information with CWOP that I forgot.

Beaudog · « **Reply #20 on:** September 06, 2012, 09:53:49 AM »

Under settings >CWOP you need to tell it to report QNH.

KenD · « **Reply #21 on:** September 08, 2012, 09:58:18 AM »

Quote from: beaudog on September 06, 2012, 09:53:49 AM

Under settings >CWOP you need to tell it to report QNH.

I don't see that option in WuHu.

Beaudog · « **Reply #22 on:** September 08, 2012, 02:27:14 PM »

Lookee here.

KenD · « **Reply #23 on:** September 15, 2012, 10:44:28 AM »

Quote from: beaudog on September 08, 2012, 02:27:14 PM

Lookee here.

Arrgh! Thought I had scoured everything but found it under "tweak." Thanks!

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Author Topic: Help with Barometer (Read 4940 times)