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ALT/BARO help for all
This is a discussion on ALT/BARO help for all within the Suunto Core & Suunto Lumi forums; I'm not insulting anyone's intelligence here, but I after surfing reviews and forums, I wanted to add my two cents ...
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06-28-2009, 02:38 PM
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 ALT/BARO help for all I'm not insulting anyone's intelligence here, but I after surfing reviews and forums, I wanted to add my two cents about barometric altimeters for the people out there that seem to complain about accuracy. I am a US ARMY pilot and deal with barometric altimeters, PA (pressure altitude), and altimeter setting every day. #1 anything with "BARO" in it uses ambient air pressure. Unlike most basic GPS units that show "GPS Altitude" only. #2 Barometric altimeters require constant (at least daily) calibration, but is the most accurate perception of MSL (mean sea level) altitude. - "so my GPS gives me absolute altitude above sea level, why do I need a barometric alt?" ANSWER: You are climbing a mountain. Your GPS says you are absolutely 9500 feet above sea level. Meaning in a 3D environment, your absolute position is 9500 feet from 0 (sea level). The problem is, due to the current weather conditions, your actual pressure altitude is higher, let's say by 500 feet. GPS says 9500ft while actually PA is 10000ft meaning your plan to strap on O2 is off by 500ft. This could be a fatal mistake as you continue up. #3 As a US pilot, I use inHg. For every .01 of inHg = 10ft pressure alt. If your watch is set to 30.00 inHg but the actual altimeter setting is 30.25 inHg, then you are reading measurements that are off by 250 feet, until you calibrate to to 30.00 inHg. #4 Prior to doing any activity that relies on pressure altitude, you absolutely need one thing - CORRECT FIELD ELEVATION of your starting point/current location. Set the current field elevation on your watch, then for bonus points, compare the watch's barometer reading to the actual altimeter setting given to you by a weather station or some other facility (ie airport). If your watch shows 29.92 but the weather station is giving 29.90, then your watch is off by 20 feet. Visit this site, enter the city/airport or ICAO airport identifier and you can get a current weather observation including the current altimeter setting. ADDS - METARs Sorry for this being so long, but it's not fair for someone that has zero clue about barometric pressure to rate these watches with a low score solely based on so called "inaccurate altimeters". Currently, my CORE ALL BLACK is within 10ft of accuracy. My helicopter has to be within 70ft to be safe and legal.....      |
| The Following User Says Thank You to mikedeployed For This Useful Post: | ||
Tyler (06-29-2009) | ||
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07-20-2009, 02:20 AM
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 Thank you very much for all that valuable info. I just got my new Core and not being very familiar with barometric pressure, I was having minor problems trying to adjust my watch. In fact, I was mostly unsure about my settings. Your post made me realize my settings were pretty good. I also learned a lot of interesting stuff. A little note to my fellow Canadians who might read this, the Canadian equivalent of Mike's link for ADDS-METAR is to be found on Nav Canada's website. Mike, following your instructions and using public informations, I think I can say that my Core too seems to be very accurate. I live in Montreal and Google Earth says my home is at 19m (62ft). The airport (CYUL) is said to be at 118ft. That airport is to the North-West from my place and closer to the mountain, therefore, higher. That's roughly a 60ft difference and it makes a lot of sense when you know the terrain. My Core shows 0.05 inHg less than the CYUL report. That's 50ft. I guess I'm a happy camper. Thanks again! Last edited by marcjutras; 07-20-2009 at 02:37 AM. |
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07-20-2009, 02:44 AM
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| If I set my reference altitude to that of the airport, I get the same air pressure as they show on their lat report (30.12 in Hg). Nice. Last edited by marcjutras; 07-20-2009 at 02:49 AM. |
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07-20-2009, 12:23 PM
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| have I got a bad Core or is this to expect ? Hi, before I get moving about: 1/ I set the sea level pressure reference around 1008 - 1010 hpa - the correct sea level pressure : my core altitude shows 12-15 meters below compared to what it should be. If I set my Core altitude to the correct altitude, core will show 2-3 hpa higher baro pressure. Is this ok ? example: My correct altitude is 45 m above sea level and I set my core to it and the correct sea level pressure is 1009 but my suunto will show 1011-1012 hpa, is this ok ? or I set the Core to 1009 hpa - correct sea level pressure but it shows that my current altitude is 24 m but actualy is 45m. Is this OK ??? 2/ now even Worse accuracy If the pressure goes down to 1005-6 hpa and I set my Core to it My resulting altitude would be 30- 35 meters below what it should show. If I set my Core to the correct altitude, the resulting baro pressure will show 4-5hpa higher figure than it should. Is this still acceptable or should I send my core to get it serviced ? My Core can track the altitude well when travelling but cant be trusted when it comes to relating the sea level pressure reference to the resulting altitude neither the correct altitude will show the correct pressure. Is there some trick for fixing this problem similar to fixing the Core’s display contrast ?? Do I expect to much from it ? Do I use it incorrectly ? |
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07-20-2009, 12:38 PM
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07-20-2009, 01:11 PM
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| Still wrong reading Quote:
when in alti profile : correct pressure is 1007 hpa, i set my core to the sea level reference 1007 hpa - core shows 16 meters which is 29m below as my altitude is 45 m when still in alti profile : Core is set to the correct alti reference and then I look what sea level is showing in baro profile for my altitude 45 m and I get 1010hpa on the Core but it should show 1007 hpa, which is 3hpa higher value than the correct sea level pressure and seting the correct figures in baro profile doesnt show accurate related values either. What is wrong or do I expect too much ? thanks for your reply Last edited by J.Core; 07-20-2009 at 01:20 PM. |
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07-20-2009, 01:41 PM
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| The altitude of my local airport is 35 m and their latest report says 30.18 in Hg. The altitude at my place, according to Google Earth is 19 m. If I set my reference altitude to that of my airport (35 m), I get 30.18 in Hg or 1023 hPa which matches the conditions at the airport. If I set my reference altitude to 19 m, I get 30.12 in Hg or 1021 hPa. That's a 0.06 in Hg difference or 60 feet according to Mike's post. The difference between 35 m and 19 m is 52 ft. That means my watch's precision is similar to mike's with an 8 ft margin on this reading. Now, I'm really no expert at this, but I believe this to normal. If I switch between alti and baro modes, the values remain the same. BTW, where do you get you 1007 hPa from? Their altitude might be a bit different than yours, just like between my place and the local airport. Hope this helps. |
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07-20-2009, 02:06 PM
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| Alti/baro mystery Switching profiles dont change values if on one spot and short space of time. My weather station in the Kitchin is pretty consistent with the official online latest charts for my area. So thats why I trust my weather station. The airport is at 63 m above mean sea level. I have to admit that recently my weather station wasnt as consistent with the airport. I guess, best thing would be to go to the airport in person and acquire the latest sea level pressure and see ... what the Core will show. I wish I could tweak the Core , like to reduce the sea level reference by 2-3 hpa without affecting my altitude on my Core ( in service or sleep mode ), then I would think the Core would calculate related values more accuratelly, is it possible ? if you know what I mean Thanks |
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07-20-2009, 02:13 PM
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| I think going to your local airport is a good idea. I'll try that myself when I get a chance. Good luck! |
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07-23-2009, 09:41 AM
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| Barometer/Altemeter tests Hi. 1/ I set my Core to the baro profile with the correct Altitude for my home location in the evening 2/ following day in the afternoon I checked the online pressure at the airport, both showed the same baro pressure 3/ I set my Core to the Alti profile - Core shows same pressure for my home like the airport 4/ driving to the airport in Alti profile, road leading me to the google maps' altitude spot was closed for security reasons but I got fairly close and the land there looks even. Core shows Correct altitude but 5/when I arrived back home I found out online that by the time I was at the airport the pressure there was 2hpa lower than my Core was originaly set to,which only leads me to the conclusion that if I add 2hpa to the correct Air pressure reference in Alti/Baro profile the Core have good chance to calculate reasonably accurate altitude. when I arrived back home the pressure must have drifted indeed as my Core showed 14 m higher altitude ( in Alti profile of course ) 5/ The sea level pressure of that day was very low 991-5hpa ( Northern Ireland ) and I wonder if the Core will require setting 2 hpa higher reference baro pressure to obtain correct Altitude on high sea level pressure days around 1028 hpa as well or if the Core will require seting it 2 hpa Lower in this instance ? 6/ If I look in the map and set the alti reference, the Core will be spot on with the altitude in Alti profile when moving about but calculating it from about 2 or maybe 3 hpa higher sea level pressure than it actually is but I dont know how the Core will work on High sea level pressure days. Do you experience something similar or will some other Core be more accurate ? Or does the Airport provide wrong sea level pressure for some reason ? Thank you for you opinions Regards Last edited by J.Core; 07-23-2009 at 09:45 AM. |
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07-24-2009, 05:39 AM
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| I believe all Core watches are created more or less equal within a certain error margin. I don't see why they would differ so much from one another. I'm also convinced the airport's instruments are way better than our watches in every aspect (quality, precision, sensitivity, etc.). Those two facts put together should explain why there are little differences in our readings and what the official reports say. Last night, I did a little experiment on my way to work. I checked on Google Earth the altitude of my home (19 m) and that of my work place (44 m). I went to the bus stop and set my Core on altimeter and the reference altitude at 19 m. About a minute after setting the altitude, it went up to 20 m and stayed there. When I arrived to my work place, it indicated 45 m. I suspect that extra meter was due to a change in pressure but I'm not sure. It could also be simply because of my height, my watch is at 97 cm from the ground when I'm standing. While at work, I set my watch back to barometer and the reference altitude to that of the airport (35m). When I came back home, the air pressure was 1 hPa above that from the weather report and 0.10 in Hg above that of the airport. Seems good to me. |
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07-24-2009, 01:05 PM
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I can't set Alti reference value without affecting Sea level pressure reference value, I wish I could, because this would tweak this added 2-3 hpa or lower 20-25m reading difference in calculating related values and then the Core would work fantastically. I agree with you that the airport's devices should be more accurate than the Core. But how can you set the Core to the 35m Alti reference value when your work place neither your home are at this 35m altitude and expect correct reading baffles me. What makes sense to me is you set the sea level pressure reference value matching your airport sea level pressure and your Core can calculate your correct altitude. My Core doesnt do it, if you had my Core and you set it to your Aiport sea level pressure value it would calculate that your work place is at 20 m altitude and your home at minus 3m altitude or something similar. My Core is 2008, maybe the 2009 one's dont have this problem ? Thanks for your replies Last edited by J.Core; 07-24-2009 at 01:10 PM. |
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10-11-2009, 05:41 AM
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| Frustrated I've been trying to reset my altimeter and barometer for the past two and a half hours. My Vector will not respond to either the sea pressure setting or altitude setting that I give it. More often than not, whilst changing it reverts back to the Default setting or having set the sea pressure at 1018mb, the altimeter is constantly going up and down, when I know that I'm at 30m above sea level. I am slowly going MAD. HELP! |
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