I’ve been working with parcel boundary shape files (.shp), standard satellite imagery, and Meets and Bounds (angles and distances) site maps / legal descriptions for a few weeks. Wondering if those can be accurately rendered in QGIS, down to about 6 inches? (yes the readily available satellite images are very fuzzy at a close range, but a high contrast wall, driveway, roof etc. can be easily seen to violate vector property boundaries.)

What level of error would be typical of that much zoom (down to perhaps 50 x 50 feet)

More details below:

There is the Azimuth and Distance Plugin (very popular - 46k downloads!) that allows the creation of a feature from angles and distances, however I’ve found it (for QGIS v 2+) early alpha level (no documentation, UI has features not implemented, etc. and buggy to the point of not being usable in versions 2+ of QGIS.

In Azimuth and Distance Plugin in QGIS 1.8 has less features but seems to work fine. Best of all it imports and exports from a standard text file so writing to those can avoid the tedious data entry step though the UI.

You can only construct a line feature in the plugin and that is limiting. Ideally, multiple polygons per feature are needed.

Many U.S. Counties now have .shp files available of all their property boundaries. How did these .shp files actually get created in the first place, and about how accurate are they? I’d assume they digitized the historical neighborhood blueprints (Plats) on file with the County and then hand entered textual Meets and Bounds on the remaining ones, rural ones, not on a Plat ones? That must have been a ton of hand work!

Would they have gone to the scanned original deeds and created CAD drawings (DXF format?) from the text descriptions of those? Was there any automation? Often times the local survey markers stuck in the earth are hard for people who are not surveyors to pinpoint and tag with highly accurate geo coordinates.

In many counties a grid system is used of 1 mile squares, further divided into .25 mile squares. The Point of Origin for a Plat is typically one the corners on the .25 mile squares. Would a County provide pinpoint accurate geo coordinates for those .25 mile squares? Might be great to have those as a pinpoint accurate .shp file.

I’m guessing over the typical acre property (about 200 feet square) a land surveyor working from County established fixed earth markers during the 1940s-50s-60s-70s and typically 200 foot vector segments could sight in boundary pins with a radius of around 6 inches from each node? Errors in angle measurement and distances from there might accumulate, and the entire boundary could be off by a foot or more from start to finish? In the 70s I noted a large retail store in my home town managed to be off on its structural steel and footers a full 1.5 feet! It delayed the completion of construction by several months because nothing fit. Were errors in site survey work of that magnitude fairly commonplace?

Taking the old Meets and Bounds from paper to a vector drawing file has a chance of error but as long as the original survey was high quality (for it's day) I can’t see that step as introducing much error.

However, connecting the Point of Origin and any physical land marks properly to the other map layers in the GIS strikes me as potentially difficult to get correct. Not only is the “connecting” a challenge but how about projection and zooming? Particularly, all way in (to the site plan level), it strikes me as potentially quite risky? I need to admit I know little about the different CRS and how well they scale.

Traditional CAD drawing software packages and GIS systems evolved separately over decades, each to serve their unique and traditional target audiences. Now the two disciplines seem to be becoming more integrated, particularly with on the ground surveyors using commercial grade GPS.

So, the question is not really just how accurate can QGIS be at the site plan level, it's questioning if its a good idea in general to push together the formerly distinct mapping functions?

  • Location of features in GIS can be stored with a very high level of precision but the accuracy will depend on a number of things. It sounds like the level of accuracy will mostly be limited by the quality of the input but other issues may come into play. I can't see any issues created by GIS that you wouldn't get in other packages – Sethinacan Apr 11 '16 at 15:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.