Choosing a coordinate system
The one decision everything else rests on, and how to be sure you have it right.
6 min read
Pick the grid your control is on
A project's coordinate system is the grid its numbers mean something on. Choose the one your control coordinates were computed on, not the one that looks closest, and not the one the last job used.
- In the project's coordinate settings, search the Canadian presets by name, 3TM, 10TM, UTM, provincial systems.
- Or type an EPSG code directly if you know it.
- Read the summary Backsight shows: the datum, the projection, the units and the area the system is meant for.
- Save. The setting appears immediately, without a page refresh.
NAD83 is not one thing
These are different systems
NAD83 Original, NAD83(CSRS), WGS 84 and ITRF are distinct reference frames. Backsight treats them as separate selections and will never quietly swap one for another.
Edmonton 3TM-114 on NAD83 Original is EPSG:3776. The CSRS version of the same projection is EPSG:3780. They look identical on a plan and differ on the ground. If you are unsure which your control is on, ask whoever computed it before you draw a line.
What Backsight refuses to do
By design, and with the reason stated every time:
- It will not substitute one datum for another to make a transformation possible.
- It will not assume a coordinate epoch. Time-dependent work needs an explicit epoch or the calculation is rejected.
- It will not download a transformation grid on your behalf. Missing grids are reported; installs are recorded with a checksum.
- It will not give an orthometric height without a geoid model you have selected and verified.
Every calculation reports the operation it used, the pipeline, the accuracy, the area of use and a reproducibility id, so an answer can be checked, and checked again later.
Checking a project is set up right
- Open the project's coordinate settings.
- As you choose, the form checks the pieces against each other and lists any disagreement under the heading “These settings disagree with each other”, a CGVD2013 datum paired with the HT2 grid, a geoid chosen for ellipsoidal heights, an epoch that is not a decimal year.
- Each line names the incompatibility and what resolves it. Nothing is corrected for you: change the setting it names, and the warning goes when the pieces agree.
Read next
- Heights, vertical references and geoid modelsEllipsoidal against orthometric, what a geoid model does, and how to install one.
- Projects: what they hold and how to set one upA project is a grid, a vertical reference and everything measured on them. Here is how to start one properly.
- Importing survey pointsGet a points file in, map the columns, and know exactly what was accepted.
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