How to read a utility long section (longitudinal profile)

The long section (longitudinal profile) is the drawing that tells you whether a utility network actually works in the vertical plane. The plan view shows where the pipe goes; the long section shows whether it can go there — at the right depth, with the right slope, without colliding with everything else underground. Here is how to read one.

The two scales

The first thing to notice is that a long section is distorted on purpose. Horizontal distances span hundreds of metres; the vertical differences that matter are centimetres and metres. So the drawing uses a dual scale — for example 1:500 horizontally and 1:100 vertically. The vertical is exaggerated 5× here so slopes and clearances are actually legible. Read every height off the vertical scale, every chainage off the horizontal one, and never scale a slope angle directly off the sheet.

The lines

• Existing ground line — the terrain as surveyed.
• Designed ground line — where present, the finished surface after earthworks.
• Pipe line — for a gravity sewer this is the invert (channel bottom); for a pressure main (water, gas) it is the axis. This distinction is not cosmetic: a gravity system is designed from the invert up, because the invert and its slope are what move the flow.

Stationing (chainage)

Along the bottom runs the stationing — distances measured from a zero point, often in hectometre form. Every manhole, bend and crossing is tied to a station. Stationing is how the long section, the plan and the schedule of structures all refer to the same physical point.

The profile table

Below the drawing sits the table — the part a reviewer reads first. Typical rows:

• existing and designed ground level,
• pipe invert (or axis) level,
• cover depth,
• gradient per reach (‰ or %) and reach length,
• diameter and material,
• vertical/horizontal angles,
• drop (cascade) at a structure.

Each column lines up vertically with its point on the drawing. The table is where the design is actually verified — the picture is the summary, the numbers are the proof. Where those gradient values come from, and how to defend them to a reviewer, is covered in the gravity sewer gradient design guide.

What a reviewer checks

Reading a long section critically means asking the questions a checker asks:

• Is the cover depth sufficient along the whole route (frost, traffic loading)?
• Is every gradient within the minimum (self-cleansing) and maximum (velocity) bounds for that diameter?
• Do invert levels match between the table and the drawn line at every manhole?
• Are crossings with other utilities shown with their conflict level, and is the clearance real?
• Where the invert steps down, is there a drop connection rather than an impossible kink?

From reading to producing

Once you can read a long section, producing one is mostly discipline: resolve every level, keep the table consistent with the drawing, and export something a reviewer can open without cleanup. That last part — a clean, dual-scale, deliverable DXF — is where most of the time leaks. Altivo does exactly this, step by step, and the gradient design rules are here.

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