Core Concepts

Sites and Scenarios

Sites

A site represents a physical location — a property, project area, or place of interest. It holds everything tied to that location: terrain data, boundaries, reference points, and one or more scenarios. When you create a new site, you give it a name and draw a site area (also called the Area of Interest, or AOI, in older docs and the export schema) to define its extent on the globe.

Site-level data includes your site area polygon (stored in geodetic lat/lon coordinates), the anchor point (origin for local engineering coordinates), and the terrain layer stack with whatever elevation, imagery, and 3D-model data you have uploaded for the site.

Scenarios

A scenario is a design alternative within a site. Every site has at least one scenario. Each scenario has its own entities — its own arrangement of solids, traces, boundaries, and other objects — but shares site-level data (terrain, site area, anchor) with all other scenarios in the same site.

Two types of scenarios:

Design scenarios — Mutable. These are your planning sandboxes. Create multiple to compare alternatives.
Actualized scenarios — Locked by default. These represent what actually exists on the ground. Unlock explicitly to edit.

You can branch a scenario to clone it for exploration, and compare two scenarios side-by-side to see entity and property diffs.

Entity Types

Entities are the objects in your scene — everything you place, draw, or import. SiteCAD has 8 entity types, each serving a distinct purpose in your design.

Component

c

A reusable group of entities with declared parameters. Components can be nested, forming a hierarchy. The site root (Entity 0) is itself a component.

When to use: Use components to create reusable design elements — a fence section, a building footprint, a planting bed — that you can instantiate multiple times with different parameter values.

Instance

i

A reference to a component that inherits its structure. Instances share the prototype's entities but can override declared parameters.

When to use: Place instances when you want multiple copies of a component. Change the prototype once, and all instances update — unless they've overridden that property.

Solid

s

A parametric 3D shape defined by code using the replicad CAD engine (built on OpenCascade). Solids support materials, boolean operations, and precise measurements.

When to use: Write solid code to define foundations, retaining walls, tanks, or any shape that needs engineering precision. Solids export to STEP for CAD interop.

Mesh

m

A static 3D model — imported assets, terrain objects, or visual elements. Meshes are rendered as Cesium 3D Tilesets.

When to use: Import existing 3D models (glTF, 3D Tiles) for context — buildings, vegetation, equipment — that don't need parametric editing.

Control Point

g

A survey marker with both local (ENU) and geodetic coordinates. Control points show the computed delta between their world position and surveyed location.

When to use: Place control points at surveyed positions to verify anchor accuracy, mark property corners, or reference known coordinates. A control point can also carry weather readings from your own hardware or a public National Weather Service station.

Boundary

b

A polygon that defines a zone or processing region. Boundaries are separate from the site area — they represent property lines, setback zones, or work areas within your site.

When to use: Draw boundaries for legal property lines, easements, building setbacks, or any area that needs to be delineated. A boundary can also mark a weather forecast or alert area.

Trace

t

A terrain-following path for linear infrastructure. Traces drape along the ground surface and support station-based measurement.

When to use: Lay out roads, fences, irrigation lines, utility runs, or any infrastructure that follows the terrain.

Annotation

a

A note, marker, or issue pinned to a point in 3D space. Annotations carry a type, a severity, optional media, and can be marked resolved.

When to use: Flag a drainage problem for a site visit, leave a question for a colleague, or pin a photo to the spot it was taken.

The Scene Graph

Entities are organized in a hierarchical tree — the scene graph. The site root (Entity 0) is a component at the top. You nest entities inside components to create logical groupings.

Site Root (Component)
├── House Foundation (Solid)
├── Driveway (Trace)
├── Property Line (Boundary)
├── Garden Area (Component)
│   ├── Raised Bed 1 (Solid)
│   ├── Raised Bed 2 (Solid)
│   └── Irrigation Line (Trace)
└── Survey Point (Control Point)

Key concepts:

Parent-child relationships — Moving a parent moves all its children. This makes it easy to reposition grouped objects together.
Components and instances — A component is a reusable template. Instances reference a component as their prototype, inheriting its children. Change the component, and all instances update.
Active entity vs selection — Clicking selects an entity for inspection. Activating (double-clicking) an entity isolates it for editing, changing what's visible and editable.

Coordinate Systems

SiteCAD uses two coordinate systems: geodetic (global) for site-level positioning, and ENU (local) for engineering precision. Understanding when each is used helps you work accurately.

Geodetic Coordinates

Latitude, longitude, and height — the global coordinate system. Your site area and site location are stored in geodetic coordinates so they're always tied to real-world geography.

ENU (East-North-Up)

A local coordinate system in meters, centered on your site's anchor point. East is +X, North is +Y, Up is +Z. All infrastructure entities use ENU for precise, small-magnitude numbers that avoid floating-point issues at global scale.

Anchor Point

The origin of your local ENU frame. Initially auto-set to the site area centroid when you create a site. If you have survey data, refine the anchor to a known RTK control point for maximum accuracy. Moving the anchor adjusts all infrastructure positions so nothing shifts visually.

Cesium Cartesian3

The rendering coordinate system used by CesiumJS (Earth-Centered Earth-Fixed). SiteCAD converts ENU coordinates to Cartesian3 at render time — you never need to work with these directly.

In practice:

When you place a solid, trace, or other infrastructure entity, you work in local ENU meters. The site area and site location use geodetic coordinates. SiteCAD handles all conversions automatically — you position objects in meters from your anchor, and they appear at the correct location on the globe.