Applied Spatial Analysis Using QGIS - Cadastral Boundary Mapping & Topology

Applied Spatial Analysis Using QGIS — GIS for Surveyors (2026 Cohort) · 27 Sep 2026 · 37:42 · Watch on YouTube (offline-safe)

I write this after every live session — so students who lost power or signal can catch up in minutes, and so our community has a searchable archive for the future. Week 1 we brought data in. Week 2 we created data and made it measurable. This week we made it correct.

This week's one question: "My map looks fine — so what makes it wrong?"

Why Week 3 Exists

A map can look perfect and still be legally worthless. Two neighbours share one boundary; you draw it twice, 2 cm apart, and you have just created an overlap — and now your map disagrees with the deed. Cadastral mapping is the discipline of not doing that.

The plan, live on screen:

1. Settle the theory — cadastral vs topographic: same land, different intent
2. Open last week's project, then Save As under this week's name
3. Create 3 layers in one GeoPackage: parcels, pipeline_network, valves
4. Build the Attribute Form (widgets + Value Map) so entry is fast and cannot be typed wrong
5. Set snapping up BEFORE digitizing — the habit the whole week rests on
6. Digitize parcels that share one boundary, then run the Topology Checker
7. Digitize the pipeline section by section, check it, then place valves on the intersections

1) Cadastral vs Topographic — Same Land, Different Intent

This is the question everyone asks, so we settled it first. Both kinds of survey are measuring aspects of land — natural features and man-made ones. Both can put a pipeline or a building on the map.

What separates them is not the subject matter. It is the intent of the map, and the purpose behind it.

Cadastral = legal. It answers "who owns what, and exactly where is the line?"

"The cadastral survey is about the legal aspect of the land."
"A legal map, bound by the laws… we have to show it to a real estate, we show it with our transactions, we will provide the rights."

It is bound by property rights and property law, it defines a parcel, and it is tied to the government authority that holds the land registry. Two neighbours, one common line, and the line is the product.

Topographic = descriptive. It answers "what is physically out there?"

"We're not controlling… our man-made features, the growths, pipeline, sewer lines, like buildings. You name all this, goes to the topographic survey."
"Engineering work, they don't care about the legal."

Natural and man-made features both belong here — roads, buildings, pipelines, water. And it carries no legal weight. The fastest way to tell which one you are holding: if it went to court, would the court accept it? Only the cadastral map survives that question.

2) Why Boundaries and Topology Are the Whole Point

In cadastral work the boundary is the product. Not the colour, not the label — the line. So the rules are non-negotiable:

• No gaps between adjacent polygons
• No overlaps between adjacent polygons
• No slivers and no self-intersections
• Edges must be coherent inside the layer and consistent with the other layers

"Every point is snapped to the other point in the same layer, on the other layers. Because this is how topology — for integrity between the features themselves in one layer, or with the relation to the other layer."
"If you have any prior, your data set, special parts of polygons, it's not closed, there's a gap, there's some slivers, somewhere. You're not able to control all of the analysis correctly."

That is what topology buys you. Two neighbours drawing the same 5 m boundary with snapping on produce one edge. With snapping off they produce two edges that disagree — and neither one is your map's problem, it is a legal problem.

3) Open Last Week's Project — and Save It As Today

We opened the Week 2 project, then used Save As to give the session its own file. Nothing we do this week can damage last week's work. Do this every week for the rest of the course: a graded project you overwrote by accident is a week you repeat.

4) Three Layers, One GeoPackage

We built three layers for one realistic job — a piece of land with a pipe network running across it:

All three went into one GeoPackage, not shapefiles. A GeoPackage is modern, keeps everything in one file, and has no 10-character column-name limit — which is quiz question 3 for a reason. One setting that matters before you draw anything: make sure the layer CRS is metres — "Make sure meters because we need to calculate the area right."

5) The Attribute Form — Stop Typing, Start Choosing

This was the biggest workflow upgrade of the week, and the first time we used it in the course:

"We didn't do this for the last few weeks. We didn't do the same thing. So by now you have to master this."

Right-click the layer in the Layers panel → Attribute Form (or set it in the layer's Attributes Form settings). The settings that earned their keep:

• Auto generate → Drag and drop. You drag fields into the form instead of hand-building it. It also keeps the field list and the form layout in step.
• Widget Display. Choose the widget per field — a text box, a dropdown, a range, a date picker.
• Value Map. Give a field a fixed list of allowed values, so the person filling the form picks instead of typing.
• Form Layout → Range. Constrain a numeric field (start at 1, step up) so nobody types 400 by accident.
• Constraints → Not null. A required value cannot be left blank.

And the reason to bother with any of it:

"This is for speeding up. I'm minimizing your errors while you typing in."

A dropdown cannot produce "Pvimc" when you meant PVC. That single change is worth more than it looks.

Three details worth copying into your own project:

• surveyed_by used a Value Map with groups. You pick the group first, then the person inside it. A long crew list stays manageable, and the value is still constrained.
• surveyed_date got a "Now" button. The timestamp is captured for you instead of typed — "We track every change in the database to the second."
• Not null and min/max were switched on, so a required value physically cannot be left empty.

6) Styling — Make It Visible Before You Draw It

Before any digitizing we styled all three layers so they could actually be seen and clicked: parcels as a solid fill, the pipeline as a thick line, the valves as obvious points. Digitizing an invisible feature is guesswork.

7) Snapping — Set It Up BEFORE You Digitize

This is the single most important habit of the week, and the instructor was blunt about it:

"This is very critical on a special cadastral survey."

Project → Snapping Options (or the magnet on the digitizing toolbar). Turn on:

• Snapping to vertices — the essential one for shared boundaries
• Snapping to segments — needed for lines and pipes
• A tolerance big enough to grab the shared vertex, small enough to stay honest

The rule that saves the whole week: set snapping up first, then digitize. If you draw first and snap afterwards, you have already created the exact gaps and overlaps that topology exists to prevent — and you will fix them by hand, feature by feature, instead of never making them.

8) Digitizing the Parcels — One Shared Line

We selected the parcels layer, toggled editing, and drew the first parcel. Then the second one started on the same snapped vertices as the first — "But in this house it has to be a common line between them." Right-click inside the finished polygon and the Attribute Form from step 5 opens — owner, surveyed_by, surveyed_date, remarks — then save. The boundary is stored once, shared by both parcels. Exactly what a cadastre does.

9) Topology Checker — The Rules and the Verdict

The Topology Checker is the tool that proves it. We set the rules explicitly rather than accepting defaults:

Then we ran validation on the parcels and got no errors — and the important part is why:

"No errors. Why no errors? Because we use the snap and we snap right on the edges."

That is the payoff of step 7. Clean topology is not luck, it is the visible result of correct settings. If your checker reports errors, go back to snapping, not to the checker.

10) The Pipeline — Section by Section, and Why the Dangles Stay

We drew the network as separate sections, one at a time, start point to end point, with each end snapping onto the previous:

• Section 1 — PVC connection, size 75, pipe_001, active, surveyed_by group 1
• Section 2 — PVC connection, size 75, pipe_002, active, surveyed_by group 2
• Section 3 and onward — same pattern, each its own feature and its own attributes

We ran the checker again. The result was clean, and the dangles at the ends of the pipeline were left deliberately.

This is the nuance worth understanding rather than just copying: a dangle is a line endpoint that touches nothing. A gap is two lines that should meet but don't. A pipeline that begins and ends somewhere is supposed to have endpoints — so its end dangles are correct data. A gap between two sections that should be joined is a genuine error. The checker can flag both. It cannot tell you which one you meant. Telling the difference is the surveyor's job.

11) The Valves — Sitting Exactly on the Intersections

Then we placed four valves on the pipeline intersections, snapped to the line:

"You go zoom in, you see this right on top of the valve… right on top of the pipe, so it's not in the air or not in the space."
"The trick here is you have to create each section by itself from start to start, and that's where the valve is."

Types recorded: gate, butterfly, through. Saved out of edit mode and ran the topology check across all three layers — parcels clean, pipeline clean, valves clean. The habit to steal: always zoom in and verify the valve is on the pipe. A point placed without snapping sits in empty space, and nothing in the attribute table will tell you so.

12) Recap — the Five-Step Version

1. Set up the layers and the Attribute Form first — decide your fields before you draw anything
2. Turn snapping on BEFORE digitizing — vertices plus segments
3. Draw the shared boundary ONCE, and start the neighbour on those same vertices
4. Set the topology rules, then run the checker — no gaps, no overlaps on polygons
5. Zoom in and prove the valves sit on the pipes

Minutes are better spent clicking QGIS live than re-reading the theory. Replay the demo at 1.25x if you missed a click.

Common Hang-ups and Fixes

1. The checker reports gaps or overlaps on the parcels: snapping was off, or its tolerance too small, when you drew. Re-draw the shared edge with snapping on — or fix the vertices by hand, then re-run.
2. A valve "floats" above the pipe: you placed the point without snapping to the pipeline layer. Enable snapping to the segment and re-place it. Zoom in to confirm.
3. The attribute form shows a free-text box when you expected a dropdown: Auto generate is still on Basic. Switch it to Drag and drop, then set the widget under Widget Display.
4. A pipeline section will not snap to the one before it: the sections are in different layers, or snapping is limited to vertices and you need segments as well.
5. Dangles flagged — do I care? for a network, end dangles are normal and correct. Gaps and overlaps are never acceptable. Know which one you are looking at before you "fix" it.
6. The project from last week is gone: you digitised without doing Save As. Reopen the previous file and redo it — which is exactly why Save As is step one of every week.

What You Should Have Now

• Week 2 project reopened and re-saved under this week's name
• One GeoPackage with three layers: parcels (polygon), pipeline_network (line), valves (point)
• Attribute Form configured on all three: drag-and-drop layout, Value Map dropdowns, range + not-null constraints, and surveyed_date with a "Now" button
• Snapping configured (vertices + segments, sensible tolerance) — set up before digitizing
• Parcels digitized so the shared boundary is a single edge, attributes entered through the form
• Topology Checker rules set: parcels = no gaps + no overlaps; pipeline = no gaps; valves checked
• Validation run clean on all three layers
• Four valves sitting on the pipeline intersections, snapped onto the lines
• Screenshot of the clean Topology Checker result posted to WhatsApp Course_Helpers

Naming note. The graded deliverable for Project 2 is the layer named parcels in EPSG:32636 — the same name used throughout the live session, the lab guide and the rubric. Keep it exactly that when you submit (right-click the layer → Rename, or rename the table inside the GeoPackage) so your file matches what is being marked.

Chapters

00:00  Welcome & this week's mission
03:05  Cadastral vs topographic - legal vs natural & man-made
08:45  Boundaries, property lines & why topology
09:26  Open last week's project + save it as today
12:31  Create 3 layers - parcels, pipeline network, valves
13:00  Attribute Form - widgets, drag-and-drop & Value Map
20:12  Valves layer, field constraints & styling
22:50  Snapping - set it up BEFORE you digitize
26:00  Digitizing parcels - one shared boundary
29:04  Topology Checker - the rules & the verdict (no errors)
30:40  Digitizing the pipeline, section by section
33:54  Pipeline topology, dangles & placing the valves
36:19  Recap, the deliverable & what's next

What's Next

Week 4 — Attribute Queries, Expressions & Selection Math (Project 3): we stop drawing and start interrogating the data — attribute tables, SQL expressions, AND / OR / LIKE / IS NULL, filtering and selecting features, then exporting only the rows you need. "Give me every primary road in Sudan — without scrolling 455,000 rows by hand." Bring your three layers from this week; we will query them directly. Every Sunday 9:45 a.m.

Resources: YouTube playlist Applied Spatial Analysis Using QGIS — Live Sessions & Labs · Video: Week 3 on YouTube · Course site saltco.org · Software QGIS (free & open source, offline) · Previous: Week 2 — Permanent Metric Transformation · Full publish kit in recordings/zoom_recordings/03_WEEK3/

Did I miss something? Tell me in the Q&A or open an issue — this archive is living and I will update it. — Sallah