Arc / Curve Measurement

The Arc / Curve tool measures curved features on your site such as road curves, roundabouts, kerb returns, and circular structures. It supports two input modes — define an arc by its centre point, radius, and bearing range, or click three points on the curve and let SiteView calculate the arc for you.

Activating the Tool

Open the Measurements tab in the site detail floating panel and click the Arc icon, or press R on your keyboard. A mode selector appears letting you choose between the two input modes before you start placing points.

Input Modes

Centre + Radius + Bearings

This mode defines the arc geometrically from its centre point:

1. Click to place the centre point of the arc
2. Enter the radius in metres using the input field in the results panel
3. Enter the start bearing — the compass direction from the centre to the start of the arc
4. Enter the end bearing — the compass direction from the centre to the end of the arc

SiteView draws the arc on the map and calculates the results. This mode is ideal when you know the design parameters of the curve — for example, from a road design plan that specifies the centre point, radius, and tangent bearings.

💡 Tip

Use this mode when you have the curve geometry from a design drawing. Road and rail design plans typically specify curves by centre, radius, and tangent points — enter those values directly to verify the as-built curve matches the design.

Three Points on Curve

This mode calculates the arc from three points you click on the map:

1. First click — place a point at or near the start of the curve
2. Second click — place a point at the midpoint or somewhere along the curve
3. Third click — place a point at or near the end of the curve

SiteView fits a circular arc through the three points and calculates the centre, radius, and all derived measurements. This is the easier mode when you can see the curve on the map (such as a road kerb or a roundabout) but do not have the design parameters.

ℹ️ Did you know?

The three-point method fits a perfect circular arc through the three points. If the actual curve is not a perfect circle (for example, a clothoid spiral or a compound curve), the fitted arc is an approximation. For complex curves, consider using the Alignment tools which support spiral transitions.

Results

Both input modes produce the same set of results:

Visual Display

The arc is rendered on the 3D globe as a smooth curve between the start and end points. Additional visual elements include:

• Arc line — the curved measurement line drawn on the terrain
• Chord line — a straight dashed line from start to end showing the chord
• Radii lines — dashed lines from the centre point to the arc start and end (visible in centre+radius mode)
• Centre marker — a point marker at the centre of the arc

These visual elements help you verify that the arc matches the feature you intended to measure.

Practical Uses

• Road curves — measure the radius and arc length of horizontal curves to verify design compliance
• Roundabouts — measure the central island radius and the circulating carriageway width
• Kerb returns — measure the radius of kerb returns at intersections
• Curved structures — measure the arc of curved retaining walls, seawalls, or building facades
• Pipeline bends — calculate the arc length of curved pipe sections

💡 Tip

When measuring a road curve using three points on curve, place the first and third points at the tangent points (where the curve meets the straight sections) and the second point at the midpoint of the curve. This gives the most accurate result for a simple circular curve.

Accuracy Considerations

The accuracy of arc measurements depends on how precisely you can identify the curve on the map:

• With a processed orthophoto — you can clearly see road edges, kerb lines, and other curved features, allowing accurate point placement
• Without an orthophoto — you are limited to satellite imagery, which may not have sufficient resolution to identify curve details

For critical measurements, zoom in closely and use the orthophoto overlay to place your points precisely on the curve.

⚠️ Watch out!

The arc tool assumes a perfect circular curve. Real-world road curves often include transition spirals (clothoids) at the entry and exit. Measuring across a spiral transition with the circular arc tool will give approximate results. For full alignment analysis including spirals, use the dedicated Alignment tools.

Saving the Measurement

Click Save to record the arc measurement. Add a category, notes (such as the road name or chainage of the curve), and photos. Saved arc measurements appear in the Measurements tab and can be re-displayed on the map.

What's Next?

• Distance & Bearing — measure straight-line distances between points
• Area Measurement — measure the area of a polygon
• Measurement Tools Overview — return to the full list of measurement tools