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Referencing in Pixpro

Referencing in Pixpro

Referencing in a photogrammetry enables the proper scaling and/or the asbolute positioning of the project. Without any scaling information photogrammetry cannot be used for measurements. In Pixpro there are a few different ways controlling referencing of your project.

The first step of controlling the referencing system of your Pixpro project is to select the coordinate system when creating the project. This applies to all types of referencing workflows. Project creation step is described in Layer creation section of the documentation.

Types of referencing in Pixpro

Photo referencing

If the images do contain GPS information, depending on that information the project will have scale information. However there are few grades of accuracy regarding the embedded GPS data.

  • Standard GPS information of consumer drones - this is the most common and the least accurate type of embedded GPS data. Such data is provided with all currently available consumer drones. Measurement accuracy of such data is good for distance measurements of up to 1 centimeter accuracy, and volumetric measurements of up to 2 percent accuracy in regards to real world. Volumetric measurements are usually less stable when using consumer grade GPS data, and overall Z axis position of the project is usually way off the real world surface levels. This means that absolute height of a project made with consumer grade GPS data will not be represented accurately.

  • RTK GPS information - Real Time Kinematic GPS data offers much higher accuracy of the embedded GPS data. Usually such data will ensure centimeter level accuracy of the measurements in the software and will improve volumetric measurements over the consumer grade GPS data. In addition the absolute position of the project will be much more accurate including the Z axis. RTK level data is often good enough for all types of medium sized projects for professional measurement workflows.

  • PPK GPS information - Post Processing Kinematic GPS data offers the best accuracy of embedded GPS data that is available today. This level of accuracy will ensure precise project positioning even with large scale objects such as road works or huge open pits. The absolute position will be as good as possible and the measurements will only be impacted by the quality of reconstruction itself.

The accuracy of the GPS data contained in photos do not alter the workflow in Pixpro. Pixpro will utilize accurate and standard GPS data.

Photo GPS data based referencing is the easiest way to get scale and postion data for your project. After creating the project and selecting the appropriate coordinate system, photo input step comes next. GPS data contained in the images will be detected automatically and used for referencing.

If there is a need to use only a part of the image set for referencing images can be selectively filtered. To select images for referencing go to Referecing menu and select Referencing.

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In the first section of this window click on Select Photos. A new window will pop up:

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Here you can select the photos which will be used for referencing during the reconstruction. Uncheck images from the list which are undesirable for referencing and press Ok. Only the checked images will remain in the referencing calculation for all further processes. You can alway come back and change this parameter in the same manner.

Referencing by GCPs

Ground control points can be added before or after the 3D reconstruction. These points should always be of survey grade accuracy and generally do provide the best overall referencing accuracy in photogrammetric workflows. The GCPs can be used in conjunction with embedded GPS data of any accuracy to improve upon it. The GCPs can be made with real world geographic coordinates or with accurate arbitrary coordinates if the absolute positioning is not needed.

To add GCPs before the actual processing has begun begin by creating the project and selecting the appropriate coordinate system. Load the photos using the photo manager and press OK. Once the photos are loaded into the project, we can import the GCPs. GCPs can be imported by going to the GCP Manager in the toolbar or pressing right mouse button on Ground Control Points layer in the layers panel and selecting Import GCP. Select your GCP file and make sure that all GCPs fall in thei correct locations in the map view.

On how to use GCPs after the 3D reconstruction is made go to Ground control point workflow page.

Flight tracks

Flight tracks are essentially the same type of georeferencing as embedded GPS imagery data, but if the data is physically written in a separate file, rather than the images itself. Pixpro has the ability to use flight track data from a .txt or .csv files without any additional software.

On how to use Flight tracks see Flight track import.

Other referencing features

Creating your own reference grid and using GCPs.

In Pixpro you can create a custom grid (also known as local grid, mining grid, calibration grid). Custom grids are reference systems which are calculated by applying translation, rotation and scale constraints to an origin point. These grids are mostly used by professionals in mining, surveying, construction areas to ensure the highest accuracy levels in the local project space.

A custom grid must be created before creating a project.

To create a custom reference grid, go to Tools tab in the top menu and choose Coordinate systems.

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A new window will appear prompting the user to choose a base system. The base system should be the same as the system on which the custom grid measurements were acquired during the data collection. Choose the correct coordinate system from the list and press Add button. NOTE: if the deprecated status of the selected system is Yes, you might not be able to use such a system for long. as deprecated coordinate systems might be removed with future updates.

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In the next window the user is prompted to enter the name of the new system. After inputting the name of the system press Set Local Grid button. From now there are two different ways of setting the local grid parameters:

  1. By directly entering translation, rotation, scale and origin point values, in this window:

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  1. By entering two points of the custom grid with their equivalent GPS based points. To do that, press calculate and a new window will appear:

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In this window enter Reference Point 1 and Reference Point 2 data as prompted by the software. Projected X and Y would be the GPS based point data while Local Grid X and Y are the local grid equivalent values. After the values are entered the transformation data will be calculated.

After creating the custom grid system, press OK and the system will be saved in the list.

Now a project can be created. Select the custom grid system as your projects' coordinate system and begin the usual workflow. With the system correctly created and selected, overall workflow of the project is identical. Any imported GCPs will be usable in the correctly created custom grid system.

Orientation flip in arbitrary projects

If a project is already created and has no referencing data (arbitrary referencing system) I.E. it was created with a handheld camera with no gps data - one option is available which enables changing the orientation of the entire project if the initial estimate was incorrect, for example the scanned object is on its side or upside down.

This option can be found in Referencing menu tab - Orientation Alignment. With Orientation Alignment function - whole project can be flipped using a chosen axis/plane in this window:

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Click on any of the letters on the diagram to flip the entire project on the represented axis. This may take a few tries to get used to, but the project can be flipped in any direction any number of times. If for a reason, a referenced project is being flipped, all referencing will be cleared before project flipping.

Changing the Projects coordinate system

It is always recommended to choose the coordinate system of a project correctly when the project is created, however the ability to change the coordinate system after the project has been created is available.

Go to Referencing tab in the top menu, and choose Coordinate System. Select a new desired New Coordinate System from the list and press OK.

Be careful when selecting a new system, old and new coordinate systems must overlap and have logical conversions.