Using the Advanced Digitizing Panel in QGIS to input distance into UTM layer

Question

I am trying to input (survey) line distance accurately using the Advanced Digitizing Panel. I am working in a layer with a UTM CRS and entering distance based on grid measurements. Unfortunately it seems the program is applying a scale factor but it seems to be applied backwards and I end up with a slightly larger number when it seems that if the program assumes I am entering Meters and applies a conversion to grid measure then it should be slightly shorter?

Ideally I would like to enter an overland measurement and have the program apply the correct scale factor for a local grid measurement. Is this possible?

In the image below I have entered d = 4.254 and used a python label of " round($length,3)||' m' " also to show the issue with all available decimal places " $length "

Keep in mind my knowledge of Python, and QGIS for that matter, is limited.

Answer 1

This is an area where cartographers and surveyors use terminology that is not fully aligned. There are actually 3 distances, shown on this diagram (which I have borrowed from https://community.myrockpile.com/t/what-is-the-difference-between-grid-and-ground/755, which also provides a partial explanation, and annotated in red)

What is shown on a survey (plat) is either ground distance or grid distance. There is generally a note on it saying which one, and giving a combined scale factor (csf) to convert between the two. This csf is a function of the (average) site elevation, relative to the ellipsoid, and the ellipsoid + geographical position given (in this case embedded in the UTM projection). For examples of the calculation see https://www.reddit.com/r/Surveying/comments/yoalwi/combined_scale_factor_calculation/ (reddit link, may go stale) or https://www.bluemarblegeo.com/knowledgebase/calculator-2020sp2/Scale_Factor_and_Convergence.htm (less informative but static).

What is used in GIS is either planimetric distance, which is the same as surveyors' grid [Edited: I originally mistyped 'ground'] distance, or ellipsoidal distance, which would be the measured distance along the earth's ellipsoid if the elevation were zero.

The advanced digitizing dialog uses planimetric / grid distance (see QGIS Advanced digitizing switch from Cartesian to Ellipse). You should either enter the distances as notated on the survey, or multiplied by the csf provided on the survey, based on whether the survey says distances are grid or ground. If you continue to use the same CRS, this should match exactly the length($geometry). In contrast, as others have said, $length will return the ellipsoidal distance, which will be very slightly greater.

The difference between ellipsoidal distance and grid/planimetric is largely a function of the subtended arc of the earth surface, i.e. the overall distance being measured. In your instance, you have seen that it makes for a difference of about 1 part in 5000 at that scale.

The remaining difference to ground distance is largely determined by elevation in the region in question.

Answer 2

Be aware that there are two ways to measure distances: planimetric (cartesian) and ellipsoidal (respecting earth's curvature as modelled by the ellipsoid used in the CRS).

Planimetric lengths are calculated with length($geometry), ellipsoidal lengths with $length. When you use the in built measure line tool from the toolbar, you can choose which kind of measurement you want to use.

If you use UTM zones, inside the valid area of the CRS, differences should be minimal. In your case, we speak about 1 millimiter (!) distance in real world. Apart from very specific use cases where you need sub-mm precision like high-precision measurements in engineering (but then, you would be a specialist in the domain and know exactly what you need), such differences can be ignored.

Avoid overkill with meaningless pseudo-precision and make your numbers better readable. In this context, see also this answer about the connex between accuracy and precision and at what scale precision makes sense and when not: https://gis.stackexchange.com/a/8674/88814

I tested it with a line of your distance (ca. 4 meters) as well with a line of ca. 1615 km inside the valid of the CRS: see following screenshots:

Line in Toronto with EPSG:2958. See the difference between planimetric and ellipsoidal length measurement. As you can see from the street background, the difference is negligible:

A line of more than 1616 km - the difference here is less than 600 meters:

Answer 3

Just to make things complete: if you have to draw lines with exact distances/angles in QGIS, you can also use Geoemtry Generator or Geometry by Expression. It uses planimetric distances. On the screenshot, I drew a line rectangular from the initial black line, starting from the end point of the black line to a point 4.254 m away, see measurement tool. The expression uses project() function:

project (
    end_point ($geometry),
    4.254,
    radians (degrees (main_angle ($geometry)) -90)
)