[Steven Callaghan]

Texas has five state plane zones…with sp 1 the most northerly zone, sp 5 is the most southerly…knowing the location of the project, which county/city it is in, will help to locate it in the appropriate State Plane zone…there are ways to match up unknown projections to lat-long, but that discussion is way beyond the scope of this discussion thread.  Also Texas several UTM zones, running east-west.  FYI, Garmin etrex accuracy varies from 10 to 300 feet depending on the techniques used to collect the points, time of day, satellite configuration, etc.  This is not a relative accuracy, but individual point accuracy…this is based on comparisons made using data from Leica 1200, GPS+Glonass, horizontal, subcentimeter survey grade equipment, using post processing GPS software, and comparing to the same points collected using a garmin handheld.  The handheld garmins appear to be subject to an affect that could be described as a “bounce.”  This drops your level of accuracy due to a variety of phenomenon, including vertical obstructions, bad satellite configuration, lack of available satellites, etc. What happens is your accuracy is greatly reduced, even though the etrex may predict/display an accuracy of plus/minus 20 feet, but your real accuracy due to this “bounce”  for the collected point is plus/minus 100 feet.  Once again, this is getting beyond the scope of this discussion thread. 

[Steven Callaghan]

How many points are you converting?  100, 200, 5000????  The etrex collects data in Latitude-Longitude, WGS84 only…the data can then be converted from the coordinate system to any other coordinate system on the fly in the etrex…btw this is the abbreviated explanation.   However, the etrex is not very robust when converting coordinates… NADCON is the standard software used by the US government for converting coordinates from Lat-Long to NAD83…However you need 2 more pieces of information…the AutoCAD drawing is probably in a NAD83 State Plane Zone Projection.  It is also in a “unit.”  This unit is feet, meters, etc.  You do not mention the State Plane Zone in your post, you only mention NAD83.  Find out what the drawing units are in the AutoCAD drawing.  Pure NAD83 uses GRS 80 Lat-Long, essentially a spheroid…difference between GRS80 and WGS84 is insignificant for what you are doing…NAD83 projected coordinates, aka State Plane Zones, convert Lat Long to x,y, or northing, easting using lots of mathematics.  NADCON converts your lat longs to the appropriate State Plane Zone in the units you choose, feet or meters.  ESRI ArcGIS uses this in the background.  However there are more rigorous and accurate ways of handling the conversion other than using NADCON…NADCON needs to be configured for the appropriate coordinate system and format, degrees, minutes, seconds, or decimal degrees for the input of the lats and longs, and then the output coordinate system needs to match your AutoCAD drawing, and an example would be NAD83, Alaska State Plane Zone 4, US Survey feet.  NADCON has the option to batch process a text file or individually convert a single point,  if it is less than 100 points, might as just individually convert each one.  

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