Adding GCPs to Pix4D software

Introduction

Last week we learned how Pix 4D is a powerful tool used in creating excellent orthomosaics and 3D models using geotagged images from the Canon SX260 camera and GEMs imagery with imported GPS coordinates. This week we are using our data collected during our fourth field activity and using ground control points or GCPs to increase the geographic and aesthetic elements of the created geo-referenced mosaics and models.

Summary of Methods

For data collection details refer to link above, but in review: Data was collected using a Canon SX260 camera with CDHK software installed so images could be taken at set intervals over the course of the mission. Before the mission began, 6 GCPs were placed around the study area and the high precision Topcon Positioning System was used to mark each GCPs latitude, longitude and altitude. The Topcon data was saved to a text file to be later imported to Pix4D. The SX260 was mounted to the Matrix UAV to take nadir imagery and a waypoint data was uploaded to the Matrix via Mission Planner. Data was then uploaded to a shared file to be analyzed.

There are multiple ways to add GCPs to a project in Pix 4D. Those being:

1. Measuring in the field with topographical equipment much like we did
2. Taking GCPs from currently existing geospatial data and finally taking GCPs from a web map service. So you don't necessarily need your own GCPs, however your final project will be a lot more accurate if you mark
3. Through collecting GCPs specific to your project.

There are three methods for tying images to GCPs. The first case is the one that we ran and requires that the GCPs have a known coordinate system. Here you can add GCPs to your project, run an initialization, mark the GCPs in the ray cloud and then complete the final two processing steps. The second method involves the a user who doesn't have geolocated images or images and GCPs are geolocated within a local coordinate system. In this case an initialization would be run first, followed by then adding and marking at least three GCPs with rayCloud and then adding more GCPs as necessary followed by running the last two processing steps. Finally, Pix4D allows you to, for any case mentioned in the previous two methods, to Add your GCPs Manually and mark them followed by doing the total processing all at once. This method requires the least amount of intervention, but probably won't produce optimized results in comparison to the first or second methods. More information can be found on the Pix4D support site, here.

Like mentioned earlier, our data was processed using the first method. First, our data which consisted of around 340 georeferenced pictures was added to our Pix4D project, following the same steps as last week. For this exercise we had to select a coordinate system, which for where our study was done was NAD83/UTM zone 15N, which can be found in the drop down menu. After creating the project, the text file containing our GCP information via clicking on the layers menu on the left column, selecting GCP and then Manual Tie Point Manager. It is important to know how your coordinate system is stored since Pix 4D displays them YXZ. Verify your coordinates are cooret before moving on. Once this was done, Initialization was started, making sure the other processing steps remained unchecked. This will show you your initial processing before you optimize the images to the GCPs.

Figure 1: Initial Processing Quality Report. Notice that no GCPs were used in the making of these
 mosaics as indicated by the exclamation point by the Geo-referencing check.  

Once Initial processing was complete, a quality report was generated and saved for comparison to the final processing. Now the GCPs were added. To do this, again the layers drop down menu on the left in rayCloud was clicked, followed by GCP and manual tie point manager. from here you can see each GCP you uploaded underneath. Click on the GCP you want to calibrate and on the left, a properties column will appear. Here you see your GCP that you clicked and how many images you have calibrated (should be 0 to start). Below that you will see an image similar to that of Figure 2. only you will not see the GCP marker. This is where the image believes the GCP is. To fix this, zoom out until you see the marker and then zoom back in, as close to center of the GCP as you can and click there. A yellow circle will appear indicating the new, calibrated location of the GCP.

Figure 2: GCP calibration images. Yellow circles indicate a calibrated image
and the Green X indicates the new, universal location of the GCP in your imagery

Continue to tag these images, marking the center of the GCP marker and do this for at least 10 images to solidify the location. As you move along, Pix4D automatically calibrates images which means you won't have to tag the hundreds of images that contain your GCP. Once you are satisfied with the calibration, click apply and your changes will be applied to your imagery. Do this step for each GCP and when you are done and changes have been applied, hit the optimize button which will optimize your imagery. Then when you are satisfied, click the process buton on the top menu bar and from that drop down menu, select reoptimize. Reoptimization will ensure your GCPs are utilized accurately throughout your project. Then a generate a quality report to see the changes.

Figure 3: Optimized data quality report. Notice how the study area grew in
size with the addition of GCPs.

From this point, the final two processing steps can be completed and depending on the processing power of your computer, it will take a fair amount of time.When processing is done, a map such as the one in figure 4 can be created. Plotted here are the GPS coordinates as they were recorded by several different means in a previous experiment with varying degrees of accuracy.

Figure 4: Map of study area with overlayed GPS coordinates as calculated by
various GPS enabled units

In ArcMap you can calculate the error of the various GPS systems using the measure tool in the top tool bar. In Figure 5, the distance between the most accurate GPS point and the iPhone geotagged photo is measured,

Figure 5: Measuring distance between the Topcon GPS unit and the iPhone 

Conclusion:

Pix4D remains to be a user friendly source for processing geospatial data and as shown above becomes even more accurate with the addition of GCPs. It is worthy to note that you do not need to have collected GCP data before collecing your imagery data, but do know that the quality of your project depends on the amount of work that you put into data collection, so for accurate results, collect GCPs with the most accurate means possible. Collecting GCP data is considered a must when it comes to conducting long-term surveys and I would uphold my suggestion from last week to use Pix4D for processing your data.

Pix 4D Software Review

The Software

Figure 1: Pix 4D logo

Pix 4D is a powerful software system that can convert images captured through areal photography into intricate orthomosaic maps. It is useful in a very broad range of UAS mapping applications such as management of mining extractions and monitoring crop field health with NDVI to name a few. (for more on Pix 4D applications click here). Out of the image mosaic programs I have used by far, this software is by far the most user friendly if you are new to this field. Probably the most difficult part of using this software is proper data collection. For best results, it is recommended that the imagery you collect has a frontal overlap of 75% (that is between sequential pictures, and at least a 60% side by side overlap (between passes) and without using ground control points which aren't required but are necessary for lengthy surveys, a steady altitude must be flown to get accurate elevation data. Data collection increases in complexity if you are imaging a uniform field, such as a snowy landscape or crop plot where the images collected may all look similar with out distinctive landmarks. To solve this problem, it is recommended that the UAS flight be carried out at low altitude to increase visual content with a 85% frontal and 70% side overlap and a grid images aquisition plan is used. It is also important if you are imaging a uniform field that you are collecting accurate image geolocation and the project is set to alternate processing mode. Though these can be difficult parameters depending on your equipment, Pix4D has a functionality called Rapid Check which will provide a useful, quick preview of the project reconstruction and will assess the quality of data right away so you can know how your data turned out before you leave the site. Another excellent feature of Pix4D is the ability to process multiple flights into a single orthomosaic. Multiple flight processing can be done if the individual images of both flights have enough overlap, the flights themselves have enough overlap so that they can be oriented and stitched together, and if the images are taken under (ideally) similar environmental conditions. Another great feature of Pix 4D is its ability to process oblique imagery such as the imagery we collected of the concession stand in week 5. The recommended flight path for this kind of operation asks to circle the structure first with a camera angle of 45 degrees and collecting images ever 5-10 degrees for sufficient overlap and the next flybys should be at higher altitude and a decreased camera angle. Oblique imagery cannot be used to create an orthomosaic. Now that you know about the software, let me how you how it works.

How to Use Pix4D

Step 1) Open Pix4D

When you open Pix 4D you will be prompted with the following screen. From here, go to the upper left corner and click the project button. From the drop down menu, select new project.

Figure 2: Pix4D homepage

 Step 2) Create a new project

From here, Pix4D will bring you step by step through the process of creating your orthomosaic. In the new project window, you choose a name for your project, where you want it to save, and a project type.

Figure 3: New Project information window

 Step 3) Select Image Data

The next window prompts you to add your imagery. Select all the images that pertain to your study and add them to your project. A check or X will appear by the "Enough images are selected." if you get a green check mark, you can proceed to the next step.

Figure 4: Select images for your study

 Step 4) Verify image properties

The next window you are prompted with will show you if your data is able to be geolocated and what type of device the images were taken with. If the images are not automatically geolocated, but you do have geolocation data, such as imagery taken with GEMs hardware then this data can be added under the 'Image Geolocation' heading. When you add the geolocation data to the images make sure that your latitude and longitude are correct and not flipped. You will also notice that the number of geolocated Images will increase giving you the okay to move forward.  If your images are automatically geolocated such as with the data from a Canon SX260 that I used below, then you should see all green check marks and you are ready to move on after a brief overview.

Figure 5: Set up image information

Figure 6: Setting up an output coordinate system. This can be detected automatically, but you can edit it if need be

Step 5) Select your map type

Here you can select from a variety of map types that suit your needs. Since we gathered a series of nadir images, we want to make a 3D map. However, if you collected oblique images of a structure, then you would select 3D model.

Figure 6: Selecting your map type

Step 6) Processing

When you hit next on the previous screen processing is ready to begin. The software processes in three steps including, Initial Processing, Point Cloud and Mesh, and the last step is DSM, Orthomosaic and Index. It takes a lot of processing power to fully develop the project so expect this step to take an hour or so depending on your computer's processing ability and the number of images in your study. In the meantime you can view images of the point cloud and an overlay of your flight path on a satellite image of your study area.

Figure 7: Point cloud in blank space

Figure 8: Flight path overlay on satellite image with picture locations marked 

Step 7: View quality reports along the way

As each step concludes processing, you will be prompted with a quality report. The quality report will show you copious amounts of data relating to your data and how it is being processed. Such things include, image previews, file names, area measurements and so on. This is very useful to keep an eye on your data while it is being processed and even more important after to ensure you have the quality data that you or your employer need.

Figure 9: Quality report sample

Step 8: Enjoy your orthomosaic

Once processing is complete, click on ray cloud on the left side bar. From here you can manipulate you mosaic to your liking to show the best resolution. At first you will only see a series of pixels floating in space, so to fix this you must check the box by 'Triangle Meshes' and this will give you your orthomosaic image shown below. You can also study specific areas such as a sidewalk or building on your orthomosaic by creating objects that outline the area of interest. When you do this, you will be given data such as elevation and area of the object you want to study. Another neat feature is the ability to create a 3D fly through video which could be useful in presenting data. That feature can be found by clicking the video animation button on the right of the top task bar.

Figure 10: Final processed orthomosaic 

Pix 4D files can then be transferred to a geodatabase in ESRI ArcMap in order to create maps from the processed data. You can see my maps as well as some area and volumetric calculations done with Pix4D in figures 6-8 here.

In Review

Pix4D is a highly advanced, powerful and easy to use software for creating orthomosaic imagery from UAS photography with a wide range of applications. Though the user interface is friendly, you do need to pay close attention to detail while collecting your image data making sure you follow the recommendations for proper flight altitude and image overlap for your given study. Pix4D gives you the ability to take non-geolocated images and later assign geolocation data to the images which is helpful if you have image gathering equiptment such as GEMS which doesn't automatically add geolocation data to its images. The only downfalls of this software is the amount of processing power that it needs. The larger the area you wish to map, or the more images you take, the longer it will take to process in the magnitude of hours. Pix4D will also burn a hole in your pocket many thousands of dollars deep so don't expect this program to come cheap if you want to use it for personal use. Overall Pix4D is an excellent program which is easy for anyone to use and I would recommend using it.