Adventures in GIS

Nice work!

I'm still struggling to find more time to engage effectively with QGIS,?ÿ as everyone I've worked for uses ESRI.

SERIOUSLY though, great job with your foray into the GIS arena.?ÿ?ÿ

?ÿ

The big thing that QGIS has going is its price. It also has more stuff than I'll ever learn or want to learn, but there is such a big array of shapefile base maps and specialty layers available that a lot of useful maps are within reach.

NGS makes shapefiles available for survey marks. To get the railroad marks, I selected the shapefile option, chose marks within 2 (or was it 5?) miles of downtown Greensboro, selected the railroad ones from their names and hit download. QGIS sucked them right up.

I'm learning by doing and back-engineering. Apparently, when you add a layer, you tell QGIS how to read its coordinates. That is, you tell it the coordinate reference system that the lat/lon represent. It then makes whatever adjustment it can to fit that system to the base map coordinate reference system. That pretty much guarantees that map accuracy is going to be far below survey standards, but perhaps helpful none the less.

Fascinating stuff. Undoubtedly, this world is full of nerds.

?ÿ

Posted by: @mathteacher

That pretty much guarantees that map accuracy is going to be far below survey standards

Itƒ??s been a while since you posted this, but itƒ??s not too far fetched to guess youƒ??ve only got more entangled in QGISƒ??s fantastic mapping capabilities.?ÿ I realize that your OP was probably more qualitative than quantitative, but I would love for you to expound on that last note about accuracy.

Letƒ??s say you conducted a GNSS survey and used your data collector to output your GNSS data to Northings/Eastings projected through a particular State Plane Zone.

What if you then

• opened QGIS,
• defined your QGIS-projectƒ??s Coordinate Reference System (CRS) to match the particular State Plane Zone you used in your GNSS survey, and then
• imported your survey data into that same projection.

How would that be any different than importing the data into Autocad, or TBC for that matter?

With that said, how SHOULD one properly import survey data into Autocad??ÿ Do you set Autocadƒ??s coordinate system BEFORE you import projected points??ÿ Do you import to Autocadƒ??s ƒ??No Datum - No Projection?ƒ??ÿIs this moot because everybody uses LLH?

Also, Iƒ??m realizing there are some gaps in my understanding of the data integrity after going through a chain of softwareƒ??s that may have different projection definitionsƒ?? how would I know if QGIS is using the same projection as my data collector??ÿ Has anyone wondered what the difference is between Autocadƒ??s ƒ??CA83-IIFƒ? and Autocadƒ??s ƒ??NSRS11-CA.IIF?ƒ? ?ÿIƒ??m guessing itƒ??s not very rigorous to just find the latest/greatest EPSG that sounds similar to whatever the data collector is calling NAD83(2011) State Plane, Zone X..

Just found this, (still wrapping my head around it):

https://surveyorconnect.com/community/software-cad-mapping/esri-question/#post-464871

I have definitely seen differences in a few feet between?ÿAutocadƒ??s ƒ??CA83-IIFƒ? and Autocadƒ??s ƒ??NSRS11-CA.IIF.. for whatever thatƒ??s worth.

@fugarewe?ÿ

First, while you're accomplished with QGIS, I would classify myself as a dabbler in QGIS. I haven't done much with QGIS since that post other than to import TIGER files and other .shp files for a few locations just to see what would happen.

If you look at the first picture from the original post, you will see by the warning at the top that I was not very careful in selecting the appropriate projection, and that was the lack of precision I was talking about. On the other hand, other projections in QGIS give precision notes (within a foot or within a meter) in their selection window. I haven't seen one with decimeter or centimeter precision, but then, I just dabble. Hence, I don't think that QGIS can do survey-grade mapping, but I could well be mistaken.

As to Autocad, I'm clueless. I do any graphics that I need in Geogebra, an open-source computer algebra system, or FooPlot, a function and point grapher with little computation capability. For coordinate conversions and geodesy, I use NGS tools. I'm very confident that a lat/lon position converted to State Plane with NCAT and plotted in Geogebra will be precisely located. However, there will be no background map, no roads, no trees or anything like that. Nothing but a Cartesian coordinate system that shows a point.

As to coordinate system definitions, we've had several discussions here on this topic. My first encounter with different definitions came with an old system named DNRGPS which does coordinate conversions but not maps. I wanted to modify an Iowa LDP wkt file to do North Carolina coordinates. The Iowa file was labeled as 2011, so I thought that I'd have the latest and greatest adapted to NC.

Well, I changed all the central parallel and falses and such, creating a custom .prj file. Now, DNRGPS, when it gets a custom file, compares it to its library to see if it already has it under another label. It changed my custom to "GRS80 no projection" or something like that and the coordinate conversions worked slick as a whistle.

Hmmm. So the (2011) designation is really a warning to the user that all of the coordinates you input should be of this realization. It's no fancy AI cue at all. If you enter a (1986) realization coordinate, the software has no idea that it should alter that position slightly; it just blindly and dumbly plots what you gave it.

Pardon the rambling, please. I'm amazed by the breadth and capability of GIS software, but I'm not convinced that it yet represents a perfect representation of ground-level detail.?ÿ