Re: [GRASS-user] Determining Datum From Projection Coordinates [SOLVED]
On Wed, 2 Mar 2011, Michael Perdue wrote: Hate to be a wet blanket, but both NAD27 and NAD83 UTM coordinates are supposed to be in meters. Michael, It's been quite a while but I recall data in NAD27 having units of feet. Regardless, ... Since the data have both lat/lon and some version of UTM, I think the best approach is to use the lat/lon columns to import the data, then v.proj to the common Albers Equal Area location. Rich ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] Determining Datum From Projection Coordinates [SOLVED]
On Thu, Mar 03, 2011 at 12:53:04PM -0700, we recorded a bogon-computron collision of the ru...@bogodyn.org flavor, containing: On Thu, Mar 03, 2011 at 06:15:39AM -0800, we recorded a bogon-computron collision of the rshep...@appl-ecosys.com flavor, containing: On Wed, 2 Mar 2011, Michael Perdue wrote: Hate to be a wet blanket, but both NAD27 and NAD83 UTM coordinates are supposed to be in meters. Michael, It's been quite a while but I recall data in NAD27 having units of feet. This is true of State Plane Coordinate Systems in NAD27, but not UTM. FWIW, it looks like since your lat/lons are specified at such low precision, the NAD27/NAD83 datum shift might be irrelevant. Well, maybe I'm wrong about that. I used the cs2cs utility from the proj.4 system to convert the first line of your lat/lons to UTM in both NAD83 and NAD27, and in neither case did I get what is in the table for UTM: cs2cs +proj=latlong +datum=NAD27 +to +proj=utm +datum=NAD27 +zone=11 -117.31 41.04 473943.57 4543031.77 0.00 cs2cs +proj=latlong +datum=NAD83 +to +proj=utm +datum=NAD83 +zone=11 -117.31 41.04 473944.28 4543243.74 0.00 But given the extremely coarse precision of the lat/lon columns, that's not too surprising. On the other hand, since UTMs are specified to the nearest centimeter, the reverse computation gives closer results: bogodyn: 12:59:32 25 cs2cs -f '%.4f' +proj=utm +datum=NAD27 +zone=11 +to +proj=latlong +datum=NAD27 473829.03 4543648.90 -117.3114 41.0456 0. bogodyn: 12:59:36 26 cs2cs -f '%.4f' +proj=utm +datum=NAD83 +zone=11 +to +proj=latlong +datum=NAD83 473829.03 4543648.90 -117.3114 41.0436 0. In both cases, the lat/lon computed from UTM rounds (using normal rounding rules) to what you have in the table. In the second line of the table, the UTM-lat/lon conversion is more telling: cs2cs -f '%.4f' +proj=utm +datum=NAD27 +zone=11 +to +proj=latlong +datum=NAD27 465323.37 4537116.04 -117.4122 40.9864 0. cs2cs -f '%.4f' +proj=utm +datum=NAD83 +zone=11 +to +proj=latlong +datum=NAD83 465323.37 4537116.04 -117.4122 40.9845 0. In the NAD27 case, proper rounding of the latitude would be 40.99, disagreeing with your table. Proper rounding of the NAD83 latitude gives the result in your table. So my guess based on two data points is that if you assume the more precise of your columns are the ones to take to the bank, and that the less precise was a correct rounding-off of a computation, that the UTMs are in NAD83. Of course, if the lat/lon columns are just *truncated* rather than rounded, all bets are off. You'd have to do the same computation on more lines of the table to be sure, though. HTH, T. -- Tom RussoKM5VY SAR502 DM64ux http://www.swcp.com/~russo/ Tijeras, NM QRPL#1592 K2#398 SOC#236http://kevan.org/brain.cgi?DDTNM The truth will set you free, but first it will piss you off. ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] Determining Datum From Projection Coordinates [SOLVED]
Unfortunately, there are all too many government contractors, who because there are still maps out there in NAD 27 use that as the datum and then give UTM coordinates. It can be done with many GPS units... Best, Mark Hall BLM From: Rich Shepard rshep...@appl-ecosys.com To: grass-us...@lists.osgeo.org Sent: Thu, March 3, 2011 6:15:39 AM Subject: Re: [GRASS-user] Determining Datum From Projection Coordinates [SOLVED] On Wed, 2 Mar 2011, Michael Perdue wrote: Hate to be a wet blanket, but both NAD27 and NAD83 UTM coordinates are supposed to be in meters. Michael, It's been quite a while but I recall data in NAD27 having units of feet. Regardless, ... Since the data have both lat/lon and some version of UTM, I think the best approach is to use the lat/lon columns to import the data, then v.proj to the common Albers Equal Area location. Rich ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] Determining Datum From Projection Coordinates [SOLVED]
On Thu, Mar 03, 2011 at 06:15:39AM -0800, we recorded a bogon-computron collision of the rshep...@appl-ecosys.com flavor, containing: On Wed, 2 Mar 2011, Michael Perdue wrote: Hate to be a wet blanket, but both NAD27 and NAD83 UTM coordinates are supposed to be in meters. Michael, It's been quite a while but I recall data in NAD27 having units of feet. This is true of State Plane Coordinate Systems in NAD27, but not UTM. FWIW, it looks like since your lat/lons are specified at such low precision, the NAD27/NAD83 datum shift might be irrelevant. Of course, the *best* way to determine the datum would be to query the agency that produced the data... -- Tom RussoKM5VY SAR502 DM64ux http://www.swcp.com/~russo/ Tijeras, NM QRPL#1592 K2#398 SOC#236http://kevan.org/brain.cgi?DDTNM The truth will set you free, but first it will piss you off. ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] Determining Datum From Projection Coordinates [SOLVED]
On Thu, Mar 03, 2011 at 06:15:39AM -0800, we recorded a bogon-computron collision of the rshep...@appl-ecosys.com flavor, containing: On Wed, 2 Mar 2011, Michael Perdue wrote: Hate to be a wet blanket, but both NAD27 and NAD83 UTM coordinates are supposed to be in meters. [SNIP] Since the data have both lat/lon and some version of UTM, I think the best approach is to use the lat/lon columns to import the data, then v.proj to the common Albers Equal Area location. Except that your data columns for lat/lon have far less precision than UTM, and as I pointed out in another mail, the lat/lon values at low precision convert to different UTM coordinates than the columns in the table (regardless of datum) -- hundreds of meters different, in fact. On the other hand, the much more precisely-specified UTM coordinates (expressed at centimeter precision) do in fact convert to the given lat/lon coordinates if one assumes proper rounding and NAD83 datum. It looks like the lat/lons are not the ones to trust. If you were to use the lat/lon pairs to import and convert to AEA, you're likely to have some pretty significant errors (hundreds of meters). If I were in your shoes, I'd run a test: assume NAD83 datum and use proj.4 to convert each pair of UTMs in your table to lat/lon, then round to two decimal places and compare to your lat/lon columns. If they all agree then it's a safe conclusion that the UTM coordinates are the reliable, precise ones and the datum is NAD83. Then your best bet would be to use the UTM columns to import the data into a UTM Zone 11, NAD83 location, and v.proj *those*. -- Tom RussoKM5VY SAR502 DM64ux http://www.swcp.com/~russo/ Tijeras, NM QRPL#1592 K2#398 SOC#236http://kevan.org/brain.cgi?DDTNM The truth will set you free, but first it will piss you off. ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] Determining Datum From Projection Coordinates [SOLVED]
On Thu, 3 Mar 2011, Tom Russo wrote: This is true of State Plane Coordinate Systems in NAD27, but not UTM. Tom, That looks famililar. FWIW, it looks like since your lat/lons are specified at such low precision, the NAD27/NAD83 datum shift might be irrelevant. I decided to use the lat/lon because no datum is applicable until it's projected. Of course, the *best* way to determine the datum would be to query the agency that produced the data... Er, ... I don't think they'd know. Considering the state of the flat file (an Access table), and the lack of completeness of some attributes (such as depth to bedrock), I'll go with what I have. And, these data originated with the folks who dug the wells so ... Thanks, Rich ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] Determining Datum From Projection Coordinates [SOLVED]
On Thu, 3 Mar 2011, Tom Russo wrote: Well, maybe I'm wrong about that. I used the cs2cs utility from the proj.4 system to convert the first line of your lat/lons to UTM in both NAD83 and NAD27, and in neither case did I get what is in the table for UTM: Interesting, Tom. Thanks for exploring as you did. So my guess based on two data points is that if you assume the more precise of your columns are the ones to take to the bank, and that the less precise was a correct rounding-off of a computation, that the UTMs are in NAD83. Of course, if the lat/lon columns are just *truncated* rather than rounded, all bets are off. Works for me. You'd have to do the same computation on more lines of the table to be sure, though. These data do not need to be so precise that I'll worry about it. I'll go back and recreate the table using the UTM values rather than lon/lat values, feed them through the awk script to remove missing values and the sed script to format them correctly, and try again to import them into grass. Much appreciated, Rich ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] Determining Datum From Projection Coordinates [SOLVED]
On Wed, 2 Mar 2011, Thomas D. Dean wrote: Look at http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system Some calculations should show you if it is meters ot not. tom, The magnitude of the values is also indicative; for the NAD27 values in feet they'd be much larger. Interesting article that confirms what I thought was the case. Thanks, Rich ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] Determining Datum From Projection Coordinates [SOLVED]
Hate to be a wet blanket, but both NAD27 and NAD83 UTM coordinates are supposed to be in meters. The False Easting for the UTM zones are defined to be 50m regardless of datum. So, the presence of meters in your DB is not a solid indication that it is in NAD83. As the lat/long in you're file appears to be rounded to 2 decimal places (about a km) and is undefined with regards to datum, it will be difficult (if not impossible) to determine whether the data is NAD83 or NAD27. If you can live with the ~150-300m error then it shouldn't be a problem, otherwise ... Bummer On 2011-03-02, at 5:43 PM, Rich Shepard wrote: On Wed, 2 Mar 2011, Thomas D. Dean wrote: Look at http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system Some calculations should show you if it is meters ot not. tom, The magnitude of the values is also indicative; for the NAD27 values in feet they'd be much larger. Interesting article that confirms what I thought was the case. Thanks, Rich ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
