Step 1: GCP Placement Guide

• Introduction
• GCP Placement Timing
• Surveying Equipment
• Layout Rules of Thumb
• How to Place a GCP
• GCPs for Slab on Grade Areas
• Recording the GCP Coordinates

1 Introduction

Siteaware's Digital Construction Verification system relies on the dispersion of accurately measured ground control points (GCPs) for timely and accurate verification results. The two factors that affect the quality of the scan are 1) The dispersion of the GCPs to cover the entire scan area and 2) The accuracy of the measurements themselves. This section provides guidelines for placing and measuring GCPs, and uploading the coordinates to Siteaware ensure you receive consistent results every time you scan. 

2 GCP Placement Timing

Siteaware recommends that the GCPs be placed while the surveyor is laying out control for the other trades.  This eliminates the need for the surveyor to re-establish a base point and ensures that the deck is nearly empty of construction materials, providing the most flexible layout which will lead to more accurate results.

3 Surveying Equipment

As previously noted, the tolerance of the digital twin is directly correlated to the accuracy of the GCP measurements provided. Therefore, Siteaware recommends using Total Stations to survey the control points as they can achieve the most consistent and reliable accuracy. 

If your project does not have access to a Total Station, follow the instructions in Step 5 for placing GCPs offset from the controls. 

4 GCP Layout Rules of Thumb

Once placed and measured, it is critical that the GCPs are not moved. Any movement, even if slight, can decrease the accuracy of the digital twin and subsequent scan results. To avoid such scenarios, please ensure that the GCPs are not placed in areas where elements such as sleeves or embeds will need to be placed or in locations with dense rebar or PT Tendon layouts. 

Tip: To prevent any issues with the visibility or movement of the GCPs, it's a good idea to pre-plan their location using the field-use documentation.  This will help avoid clashes with the installation and ensure a smooth GCP layout process. 

GCPs should never be placed: 

• At the bottom of a beam pocket. 
• In an area where studrails will later be installed. 

If GCPs are moved after they have been measured they must be remeasured. 

4.1 Placement Locations

To achieve the highest level of accuracy in all areas of the pour deck, it is required to place GCPs throughout the deck as each point provides a high level of accuracy for its surrounding area. At least five (5) control points (Siteaware recommends placing 6-8) per pour area should be placed. The ideal GCP layout for a typical pour area will have one (1) GCP in the vicinity of the four (4) corners of the deck and an additional GCP in the center (See Image 1). However, not every pour will be rectangular and therefore it is important to follow the following guidelines to achieve the most accurate results: 

1. A GCP should be placed close to every corner of the pour area to maximize coverage. (See Image 2 for an L -shaped deck.)  
2. If the pour area is wide but narrow (see Image 3) place the GCPs staggered as shown. 

Siteaware will also supply a recommended layout (see below) for every pour area which can be followed directly to achieve the best results. 

5 How to Place a GCP

1. Place the sticker on the deck: 
   1. If the GCPs are being measured by a Total Station, place one GCP sticker in each of the regions/locations noted above.  
   2. If placing the GCP on or offset from a control line or control point:
      1. Use the guides on the sticker to align it with the controls (see the example below).

2. Pull off the top third of the backing exposing the adhesive and press it toward the deck. 

3. Pull the other part of the back cover to expose the rest of the GCP’s sticky section and press it toward the deck.  

4. Measure each point and record its Northing, Easting and Elevation.
   1. If measuring with a Total Station, measure each point like any other control object on the deck and extract a .csv file from the data collector. 
   2. If the GCPs were placed directly on a control point they do not need to be remeasured. Send a list of the GCP numbers and their corresponding control points to Siteaware. 
   3. If the GCPs were placed offset from a control point: 
      1. Measure the actual distance to the control after the sticker has been placed. 

2. Write the measured offset distance on the deck AND on the sticker. 

6 GCPs for Slab on Grade (SOG) Areas

For scans of the slab on grade pours where a vapor barrier is being used, the GCPs may need to be placed twice.  Siteaware recommends performing the first scan before the placement of the vapor barrier to allow for the easy adjustment of any penetrations or grade beams should there be any corrections required.  Once the vapor barrier is placed these GCPs will be covered and a new set of stickers will need to be placed. 

Additionally, as the stickers are not intended to be placed on dirt, the pre-vapor GCPs should be placed on a metal plate that can be fixed to the ground to ensure they won't be moved during the scan.

GCP on a Plate on Grade:

Siteaware will provide the plates as part of the Self-capture kit

GCP on the Vapor Barrier

7 Record the Data

Once the sticker placement and measurement are complete, upload the coordinate readings to Siteaware using one of the following methods: 

1. If the locations were placed relative to the control lines, upload a .csv, .txt, or .xls containing the GCPs numbers and their locations relative to the controls. (ex: GCP 90: 10” North of C3).

GCP #	Offset
90	10” N of CP-3
91	12” E of GL C & 12

Be sure to update the GCP number to match the stickers installed in the current pour area. 

2. If the locations were measured with a total station:
   1. Extract a file from the surveying equipment and upload it to Siteaware.
   2. Take a picture of the coordinate readings from the readout or a notebook and upload the image to Siteaware.