Feb 12
8
Robert asks…
How do i get a coordination into my eTrex H GPS navigator?
I have recently purchased an eTrex H GPS navigator in oreder to do some geocaching. But the problem is that i i don’t know how to get a conversion of the coordinates given by the geocaching website to my eTRex H GPS navigator. The type of coordinates used for a waypoint on my GPS shows like this:
Elev: 455ft
ST 18629
BNG 96017
This is the coordinates that this GPS uses. How can i get a conversion from normal coordiantes to these?
world of gps answers:
Go to setup, units
change Position format to hddd mm.mmm
and map datum to WGS 84
Donna asks…
Map or Math Guru’s, Conversion Help please!!! latitude and longitude?
I have by trying to convert X, Y coordinates from a local map to decimals (lat, long) so i can enter them in to my GPS. I did numerous searches and could not find any useful information. When i click a point on the map it gives me this
X Coordinate:839055.942
Y Coordinate:468244.345
(Here is a link to the map im looking at if you want to check it out yourself http://www.tremplocounty.com/Trempealeau%20County%20Web%20Mapping/default.aspx The tool to find coordinates is in the upper right hand corner (the ruler) and that will give you a few options, click on the single dot in the box.)
Now how can i convert that to a standard N W decimal? i need a link to a conversion site or steps on how to convert those numbers because i have more points than just that one. any help would be greatly appreciated! thanks
Thanks, ill try that on a windows pc. Im on a mac now. Also, is there any mapping site that would allow me to enter those coordinates and also show me decimal latitude longitude? More sites or ideas also welcome!
world of gps answers:
Those are State Plane coordinates for your state and, possibly, a particular “zone” within your state. Your GPS needs to know what state and zone you’re in.
You may want to try this conversion software:
Follow the link below. Click on the “geodetic tool kit” button. Under State Plane Coordinates, you’ll see GPPCGP. Download the software.
Http://www.ngs.noaa.gov/
Sandra asks…
How do I convert a Datum that has been defined as “Modified (ground) coordinate system based on…”?
I am working in Autocad. I have some 100 Lat/Long Coordinates. The destination drawing defines it’s datum as such:
***
BASIS OF SURVEY:
HORIZONTAL DATUM: MODIFIED (GROUND) COORDINATE SYSTEM BASED ON THE NEW MEXICO COORDINATE SYSTEM OF 1983, CENTRAL ZONE, NAD 1983(92) US SURVEY FEET AND WAS ESTABLISHED BY 0.P.U.S. GPS OBSERVATIONS, IN ACCORDANCE WITH THE NATIONAL GEODETIC SURVEY’S SPECIFICATIONS FOR O.P.U.S. OBSERVATIONS.
VERTICAL DATUM: NAVD 1988
COMBINED FACTOR = 0.999611828
COMBINED RECIPROCAL FACTOR = 1.00038832274
***
It contains a comparison table. A comparison taken at random is such:
***
Northing – 747878.008
Easting – 1719902.164
Elevation – 4556.84
LaLatitude 33°03’17.51520″
Longitude – 105°59’33.89390″
***
I am currently using Corpscon6 to do conversions between Latitude/Longitude and New Mexico Central State Plane Coordinate system, which is what this appears to be based off of. The problem is, a raw conversion between the two creates an error of about 500 feet, which is too much of a margin of error for the use this is being put to.
My question is, does anyone know any mamathematicalormulas or anything I can do that would convert what Corpscon6 puts out to something that would be usable in the autocad datum?
world of gps answers:
The problem you are dealing with is ground v. Grid coordinates.
The UTM or State Plane “grid” is based at the ellipsoid and not at the ground surface. At the ellipsoid, 1 m on the ground equals 1 m at the grid. The ellipsoid is usually below the geoid or ground surface, therefore, 1 m at the ellipsoid is slightly more than 1 m at the geoid.
You’ll have to research these principles to understand exactly the mathematical principles.
To address your immediate issue, you are dealing with ground coordinates, but you want grid values. The ground coordinates equal the grid coordinates times a constant (ground adjustment factor).
To go from your modified state plane coordinates (ground) to grid coordinates, multiply the ground values by the combined scale factor (.9996…). To go from grid coordinates to ground values, multiply the grid coordinates times the reciprocal (1/.9996) of the combined scale factor. This is the GAF (ground adjustment factor).
If your data is Lat/Lon., use corpson to convert to SP. Then multiply coordinates by the combined scale factor. You’ll then be using State Plane grid values, which will work for you in CAD.
Good Luck.
Mike
Mapping Director
Desert Archaeology, Inc.
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