Measuring Elevation

That would all depend on where and, more importantly when, the USGS reference point was taken and calibration of equipment.Don't forget about tide too! But trust me if the USGS says your house is at x elevation, it would far harder to convince them otherwise than be greatful you have a house in the first place.

Atmospheric pressure will vary with weather as well as altitude.

Water pressure maybe from a pum rather than a water tower.

I like a good map myself..in the Uk we are fortunate that the country has been well maped by the ordnance survey and detailed maps can be bought at any bookstore or news agent. It's frustrating to go on holiday to many countries and be unable to buy a decent map.

"For an obscure reason, I want to know the altitude of my house."

From past experience "obscure reason" is readily translated to 1. Idle curiosity, 2. Just because, 3. It's a big secret and I don't want you to steal my thunder.

Get a Laser Transit and a helper and do it the old fashioned surveyors way and carry the differences in level from the reference point (A USGS Monument?) to the house site.

Don't settle for halfway improvised methods of measurement of questionable accuracy.

If you reall want to know do it right the first time and you won't have to do it over!

Ever heard the one about the GPS that directed its German driver into a lake? He got out of his sinking car and retored, "I was only following orders!"

Surveying would be the most accurate method. Example: when the Loetschberg Tunnel was constructed through the Swiss Alps the two crews tunnelled for about 17 miles from both directions towards each other until they met. The error where they did? About 1ft horizontally and 7 inches vertically. The date? 1903 <bows in admiration>!

Quod erat demonstrandum, which could be Latin for 'quite a decent result', though it isn't at all.

Water pressure difference is the old pipe bugaboo: you can't get accurate pressure unless the water is flowing, and when the water is flowing there are drops in the lines. So, it's only approximate unless there is good communication between the two points.

I think you have it backwards. The poster is trying to measure altitude difference from water pressure difference. There is a known relationship between altitude and pressure ONLY if the water is NOT flowing. If there is any flow anywhere between the two points, then the pressure changes in ways that are nearly impossible to predict.

It would be best to make the pressure measurements around 2 or 3AM, when almost no one is using water, but even then there are automatic devices and leaks...

High tech method: GPS.

Low tech method: sextant.

Did you adjust the barometer to read the sea-level pressure?

"Low tech method: sextant."

Just the way to learn something new, to him, and get experience.

"High tech method: GPS." Too easy. No learning experience.

"Did you adjust the barometer to read the sea-level pressure?" OH OH, Forgot that little detail didn't we?

I would be hesitant to use GPS for accuracy to the foot regarding elevation. GPS is not all that reliable for topo Z values.

cr3

Very low tech method : Plenty of time, helper, water filled tube (clear, open ended), measuring staff's. He/She might have to traverse several miles for all I know, but it might be fun.

I had this exact same problem, I was setting up a calibration lab for flow rate of air, so I needed to know what the exact elevation was to set out precision barometer to indicate correctly the ambient air pressure...

As Del says in the UK we have good maps indicating elevation and so once my elevation was known I could ring up the local airport control tower and ask for their spot reading of pressure and then make adjustments for elevation to set our barometer...

John.

Electroman,

I don't understand your statement. Generally, precision barometers read absolute pressure. They do not need to be "corrected" except perhaps for temperature.

Making precise flow measurements would only require knowing the absolute pressure, which is given by the barometer. No knowledge of altitude would be needed.

The only reason that you would need to know the altitude is if you did not actually have a barometer. In that case you could call the airport, get the pressure and calculate the pressure at your altitude.

Tad

Tad, for air flow measurements the ambient air pressure is needed, not the sea level pressure. This allows corrections to be made for the air density in our calibration room.

Calibrating a rotameter type of air flow meter for example with its outlet at ambient pressure we can then correct for atmospheric and altitude pressure effects to give a calibration at standard temperature and pressure...

The calibration will change if the air pressure is at 900 milibar when compared to at 1030 milibar, so we calibrate at our pressure and use the correction factor to show what the calibration would be at a standard air pressure of 1013 mbar. To do this we need an absolute pressure reference from the barometer and this can only be obtained from somewhere like an airport control tower which will be at a different altitude to us.

For example we are at I think it is 150 metres above sea level and the correction factor is -19.4 mbar relative to sea level.

John.

All interesting inputs. The obscure reason is that my wife wanted to know why the neighbors at the bottom of the hill are cleaner than we are. It is related to shower pressure. No I didn't calibrate the barometer to sea level because I was only interested in a small delta p, in this case 3.85mb. The space between adjacent USGS iso elevation lines is 3 meters and interpolating between two lines is pure guesswork. The gps method is reasonably accurate if you can get signals from 4 satellites. I haven't been able to achieve this yet. I would need more than a mile of hose to do the test with a dedicated pressure line. Is there a reason why the municipal water pipes should not provide an accurate measurement?

The position of anyone else's house won't make any difference to your water pressure..

Knowing you elevation won't make any difference to your water pressure.

Even measuring your water pressure won't effect it although it may be useful.

If you want more flow from the shower. In order of simplicity/cost

1. Clean/fit new head.

2 Fit bigger bore hose.

3 Add booster pump or raise the height of the tank in the loft if it has a water tank there (UK style).

I fully appreciate the attraction of doing displacement activities (like obscure calculations and measurements) rather than home maintenance! Perhaps you should thoroughly investigate the elevation of the local golf course.

Knowing someone's elevation won't change the water pressure, but knowing the difference between two elevations should correlate with the difference in water pressure between the two houses and explain why people at the bottom of the hill are cleaner. A new shower head is a good idea but the flow rate through shower heads is regulated by law in the US. It would be nice to find a legal method to fix this problem. I was prepared to run larger pipes in the house until I measured the actual shower flow rate (GPM) and determined that it is correct for my house pressure. The obscure measurements proved that larger pipes would not solve the problem. No tank in the loft. Booster pump is the method of last resort.

Move your house down the hill, or, alternately, why don't you shower at the other guy's house?

Many of our houses have pressure regulators in the line from the street. Unregulated our pressure is greater than 100 psi. It is regulated down to about 50 psi. However there is a fine screen in it and as it plugs you get a decided pressure drop as you run water. It starts at 50 psi and can drop another 10 to 15 psi as water flow increases.

And then some smart Aleck flushes the toilet when you are in the shower!

Hi Guest.

You have lots of answers, and some of them are OFF THE WALL.

GPS for position measurements are very accurate but this system is extremely inaccurate for elevation measurements unless you are using the WASS system where a second elevation reference correction signal is sent and received from another local location of known accuracy.

Any surveyor that has the Trimble System (special WASS system) can give you the elevation information within a centimeter or two of the known elevation.

A word on using the altimeter, you need to go back and forth several times and average the readings to allow for natural changes in barometric pressure as you do the test. A sensitive aircraft altimeter has 20 ft per division and you can read 1/2 a division. But you must handle it carefully, check back and forth several times, and make sure the temperature is the same. Just the heat of handling the unit will cause it to change. This change is small and meaningless in an aircraft but significant in your readings. There is no need to set it to sea level since you are only interested in the delta change in pressure, in fact (if it is a kollsman) setting it to sea level at at a high elevation may cause some spring memory errors in your instrument. You have to know the internal mechanics of the instrument to understand this.

Also, have you verified your water gage accuracy? I hope you are using the same gage at both ends. Any flow of water in the pipe will cause an error. It must be static pressure. The gentleman that suggested other wise is not thinking too clearly.

Hope this helps..

Snakers

Hello Snakers,

'Twas I who said pressure measurements were not accurate when done statically, and I should expand on that. If you had a large reservoir and you checked the static pressure at various levels, you would indeed get accurate measurements. And, if that reservoir had a relatively small amount of outflow, you would still get accurate measurements.

However, if you have a typical municipal water system, and you go into a dwelling and measure water pressure, you will find it useful to have perhaps 2% flow in a nearby faucet. And, yes, that does introduce error (as I said earlier). But, not doing it also introduces doubt.

All this is, of course, academic as there are no doubt regulators, pumps, and leaks to deal with. The fact of the matter is that, in order to use pressure differential to measure height, you must guarantee excellent communication between the two points.

Please go on GOOGLE EARTH it is free you can track to your house and your clean freaks down the hill. As far as them being cleaner the probably stopped using soap and started using a body wash that rinses off easier.

Feel good you live up hill.....

Don't let Google Earth fool you, I've just seen that Google earth tells me my nearby school cricket pitch has the following elevations:

South boundary: 220ft

North boundary: 188ft

West boundary: 215ft

East boundary: 203ft

Centre of pitch: 210ft

When standing on the field, it looks pretty damn level to me

Who checks for cleanliness? It might be that the downhill neighbors are bribing the judges to improve their cleanliness score. If you want to be cleaner than they, offer a bigger bribe.

Special thanks to Snaker for the relevant comments. I used a Negretti & Zambra electronic barometer that has been modified for increased accuracy. Thermal effects were taken into consideration and results averaged. The same water pressure gauge was used at both ends. It was checked against a supergage and found to be well within 1%. The water pressure measurements were off the wall because that's where the spigot on my house is located. Thanks to Miketheboilerguy for suggesting Google Earth. I had not heard of it and it worked very well. All measurements agree except the water pressure measurements(which were done under static conditions). I will check with the municipal supplier to determine if there are regulators or similar devices in the line.

Fascinating thread!

Can you quantify the correlation between cleanliness and water pressure?

Cheers -

Ever used a pressure washer?

Your wife needs to work harder so your house is cleaner than theirs. In addition, you need to pick up after yourself all the time to ease the burden on her.

So the thread has gone to why people at the bottom of the hill are cleaner.

Perhaps they enjoy showering more as a result of the greater pressure at the lower elevation? Perhaps they are closer to the shops in the town that is nearer the river bridge crossing and they nick more soap from them?

If you want to be cleaner use Zest and be Zestfully clean.

No, really. I had a water heater quit on me and couldn't get it replaced for a few days. I found out really quick that Dial, although very good with hot water, doesn't wash off well enough in cold water and gets sticky during the day. Throughout the next few days, I tried many different brands and Zest came out best by far in cold water (Florida style).

After I had the heater replaced I went back to Dial.

Maybe a recon mission to find out what brand they use would suffice. Bring along that GPS you found useless as a lovely gift.

I use Vim. It makes me scratch a bit, though.

Of course, if Zest isn't readily available, just invite Del over for the evening.

Has anyone asked 'Physicist?'?

Speaking as someone who tried to do the same thing (minus the water pressure idea) before determining it was close enough.

The GPS reading from USGS is probably the most accurate, but there grid I believe is setup on measurements every 100ft. Using a very good GPS is suppose to give you accuracy within a 90ft surface area. The Barometric conversion has way too many uncontrolled variables to trust for accuracy. And finally the water pressure difference is based off of what? Probably your GPS reading thats' accuracy was just explained.

The most accurate way would "probably" be to determine the exact location on the grid where the USGS determined elevation, then build a very large menometer from the point known point to your elevation of interest, determining the difference hence your actual elevation. Or has I said, close enough!

Hi Sonave Sunsets.

Apparently you may not be current on the latest GPS technology. As I mentioned, in a previous comment. The Trimble system used by surveyors, provides elevation data to within 1 or 2 centimeters of a known location. It requires a special second GPS unit located at a remote "known elevation site" The second transmitter sends correction data to the field measuring unit. Note this is not just an adding or subtracting system. The correction data is applied in the GPS receiver memory (on a microsecond basis) as a running average.

The Civilian version of this system is called the WASS system. The government operates a number of elevation reference stations that broadcast the correction data back up to the satellite. The higher class of newer GPS units have this WASS feature built in. Higher accuracy is obtained by making long term observations ( 1 hour or so) and computer averaging the data.

Aviation has the reference stations at each airport and can land totally on computer.

This technology has come a long way.

Snakers

Hey Snakers,

My responses were to how he was trying to accomplish the task, while jokingly throwing in the manometer (a transit w/elevation rod, if you have access to one as I do, would have been more realistic). The original question was based on a do-it-yourself approach, not hiring an engineering firm to do it for you. What fun would that be within an engineering forum?

In some areas, the water system may be divided into seperate pressure zones. Most municipal systems try to keep their pressures between 105 psi and 40 psi. As the ground drops below the reservoir, they will use pressure reducing valves to drop the pressure to the new zone. Using water pressure differences from this can result in calculating a higher elevation for property far below you!

The most accurate means of measuring differential elevation is still differential leveling with a rod and surveyors level. Unless you are using post processed data from survey GPS units, your elevations will be trash.

The barometric method basically requres a sensitive barometer and constant temperature at the two elevations. If you can measure air pressure to .01 mbar under isothermal condions, you can measure elevation changes with an accuracy of about 3.0 inches. Your home barometer can't provide this kind of sensitivity, but there are barometers that can.

Ohhhh don't make sweeping statements, my home barometer can measure to 0.1 mbar and has an accuracy of at least +/-1 mbar...

Just moving it up stairs can give a very accurate change in reading!

Plus we have one of those fequency resonant pressure transducers that cost a fortune but can measure to 0.01 mbar with an accuracy of +/-0.1 mbar ...... For this type of elevation measurement comparison, barometric change is possibly the easiest and best measuring technique.

John.

Is there a lift station between your homes, providing an extra 13.4 feet of lift, so the pressure measurement equates incorrect? Are you certain you are using the same water source?

What causes you to believe they are cleaner than you are? Do they emit a sense of superior well being and lower parasitic count or do you emotionally suffer from the lack of pressure?

Wouldn't barometer be susceptible to changing pressure zones (ie weather)? Water pressure interesting - you are measuring static pressure so no frictional loss. If it helps, for converting from one reference frame to another - google corpscon.

Yes, barometric readings are affected by changes in atmospheric pressure as well as temperature. These changes can be compensated for, however, if you pick a time for making measurements when the ambient pressure is not changing and you don't let the barometer temperature change, you can get very accurate readings. As I mentioned earlier, 0.01 mbar accuracy is equivalent to about 3.0" in altitude. Pressure gauges are made with accuracies to 0.0015 mbar, but they are not normally available to common folks. The static WP reading error has not been explained. I spoke with the local water suppliers who told me that there are no pressure regulators between my house and the bottom of the hill. I plan to repeat the test. Google Earth worked well and agrees with all measurements except static WP.