Photogrammetric Processes

Photogrammetry by definition means measurement from photographs. Here photographs being taken from an aerial platform like an air craft. These photos taken as stereo images need lots of processing before putting into actual usage. These processes are collectively known as photogrammetric processes. Stereo aerial photographs, ground control points (GCP), GNSS/IMU data and camera calibration certificates are the main input for a photogrammetric project. Photogrammetric services include processing these inputs to derive ready to use geometrically correct images. All these processing will be done in a suitable photogrammetric software platform. Aerial triangulation, bundle block adjustments, and stereo compiling are the preliminary steps of a photogrammtric process. During this process the photographs will be corrected for external as well as internal distortions. External orientation correction can be done using GCPs and tie points. Internal orientation corrections can be done using GNSS/IMU values and camera calibration certificates. Stereo viewing is possible after this stage of the processing. Now the terrain can be visualized in a 3D environment in suitable software platforms. Once the stereo compilation has done, digital surface models (DSM) can be generation from stereo models. Digital surfaces are nothing but visualization of the terrain not only on an x,y place but also with the z dimension in the form of elevation. Digital surfaces derived from the software platforms are with failed areas in many locations. These failures are due to cloud cover in images, shadows, or due to poor automatic compilation. These areas need to be corrected by editing the digital surface models. Corrected digital surface models are devoid of any collapsed areas. Ortho photos and Digital Terrain Model (DTM) can be generated using the digital surface models and stereo models. DTMs are bare earth models depicting the surface of the earth if there is not vegetation and artifacts over it. It is the true representation of the primitive earth surface. Many of the geological and other developmental projects require this sort of 3D models to know the exact and accurate height of the terrain with respect to the mean sea level. Other form of topographical maps such as contour lines, slope map, aspect map etc can be derived using these kind of 3D models. Ortho photos are another product which can be derived from these processes. Ortho rectified photos are geometrically corrected photos. Relief displacement and related distortions will also be corrected during ortho rectification process. These photos were later corrected for collapsed areas and then mosacicked to get a complete view of the area. Seam lines are commonly used for mosaicking of the images. The images thus derived can be color balanced to remove viewing of the seam lines and remove the difference to color saturation from image to image. Later images can be cut into tiles of desired dimension for easy usage. Once these kind of pre and post processing of the images are over, these will be put into use to derive information in the form of 2D and 3D feature mapping. Base mapping, land use land cover mapping and other kind of thematic mapping can be then be possible from these images.

GIS Mapping services

The modern era witnessed a change from paper maps to digital maps. As geo spatial technology evolved to the present day status, more and more paper maps have turned into digital maps. The availability of cartographic symbology, in major global GIS software has made it possible to prepare digital maps. There are different kinds of GIS digital mapping. Most common ones are conversion of paper maps to digital form and digitization from satellite images and other images using image interpretation techniques. In case of paper maps to digital map conversion, scanning of the paper maps is a necessary prerequisite. Scanning can be done to a 300 dpi level to get clarity of the features to be mapped. Then the images will be geo referenced. Geo referencing can be done using ground control points taken either from other reference maps, coordinate values printed on the maps, or though DGPS survey. During geo referencing, the map is projected to user defined coordinate systems and projection. All GIS software is having almost all of the world’s coordinate systems, projections and datum in the form library files. Once the maps are in real world coordinates, the digitization can be done using on screen heads up digitization or using a digitization table.

There are three entities in which any GIS mapping services accept the features. These are points, lines and polygons. Features like land marks, bus terminus, tube wells etc can be depicted in the form of points. Lines are loci of points with and start point and an end point. Road network, railway lines etc are usually depicted in the form of lines. Polygons are lines with start point and end points are same. Water bodies, settlements, parcel boundaries etc are usually depicted in the form of polygons. Once the features are extracted digitally in the form of points, lines, and polygons, various cartographic symbology and colors can be assigned to these features. Size and styles of features can also be incorporated at this stage to the maps. Another important advantage of digital mapping is incorporation of the attributes. Descriptions and information regarding the points, lines and polygons can be in the form of any number of attributes. In a GIS mapping, these attributes will be defined through a data base structure. Main kinds of mapping services are for geological mapping, base mapping, land use land cover mapping etc. Land parcels in the form of cadastral maps can also be prepared in this way. Administrative boundary maps, forests boundary map are also digital maps prepared in this way. Remote sensing data products such as satellite images of varying resolutions and aerial photographs are major input for preparation of digital maps. These images can be interpreted using interpretation keys such as tone, texture, association, size, shape and pattern. Useful feature extraction thus derived will be filled with attributes and then cartographic layout can be prepared to get a ready to print maps. During this process, required scale, north arrow and legend will be prepared. Overall layout map will be placed in a spatial frame work of coordinates on suitable paper size to get a ready to print lay out maps which can be exported desired formats such as jpgs and pdfs.

Challenges in Unmanned Aerial Vehicle (UAV) Data processing

The most modern development in aerial photography is the spurt of using unmanned aerial vehicles for image capturing. This has reduced the image acquiring cost to a great extent that too in extremely small time spans. The added advantages of UAV image capturing are its ability to take very high resolution images and it’s on board storage. It has revolutionized the application of UAV data usage in disaster management, precision farming, and topographical data collection of mining areas, solar farms, and border security, fishing surveillance, traffic management and many more. Though it has many advantages, this type of photography has brought its own challenges in the processing of the UAV acquired images. The major challenges posed by this technology in data processing are briefed below.
Inconsistent over lapping: This is a major challenge in UAV data processing. This will leads to non generation of stereo models and if generated collapsing of features. The author’s technical experience shows that a minimum of 80% overlap is required along flight line and 70% overlap is required between flight lines.

Distribution of Ground Control Points: Accuracy and distribution of ground control points are another challenge in UAV data processing. When comparing with conventional aerial photography, UAV require more number of Ground Control Points (GCP). As UAV data collection will be generally for small areas a dense distribution of GCPs are required for high accuracy. These GCPs should be evenly distributed and cover all the borders adequately. Accuracy of the GCPs is a defining factor in accuracy of digital terrain models and derived products.

Requirement of hardware and software: UAV data processing requires additional software to process the data. There are quite few software available in the market for processing UAV data. Each software has its own merit and demerits. Most of the software requires high end hardware to work on. This is a major challenge in data processing. As UAV swath is very small, number of images will be very high. In such instances one should require a high end hardware system with high level of RAM and very good quality graphic card, high storage space and other peripherals. 

Block to Block changes in altitudes: As UAVs are very small in size and flying at a lower altitude comparing with conventional aerial photography, causes flight line changes in altitude. This will leads to more manual editing along the block boundaries.

Failure areas: UAV data contain many numbers of images even for a small area. So, almost all software output will show failure areas much greater in amount than conventional photogrammetric methods. So a sizable amount of manual interference is required for correcting the Digital Surface Models (DSM). Data processing involves tedious manual editing of the failed and collapsed areas.

Absence of Auto Digital Terrain Models (DTM): None of the existing well established software are not providing with direct DTM. So manual editing with break lines is essential for getting very high accuracy Digital Terrain Models (DTM).

Issues in ortho photos: Another challenge in UAV data processing is the collapsed areas in ortho mosaics. Some of the software claims that they have this auto correction facility, but in practical manual interference is required. Ortho photo mosaic editing is required using Photoshop which is a manual tedious process.

SBL Geomatics provides complete solutions for all standard tasks in digital photogrammetry services such as aerial triangulation, DEM / DTM generation, photogrametry services, stereo compilation, topographic and planimetric feature extraction, orthophoto production, color balancing, mosaicking and tile generation, pan sharpening, contour generation, 3D terrain visualization, orthophotography services and LiDAR services

http://www.sblcorp.com/Geospatial-Services/GIS_Services.php

SBL Case Study : Ground water contamination monitoring through time series 3d modeling

Abstract:  The work under discussion was awarded to SBL by an environmental management organization based in USA.  The client requirement was to make a 3D model of the contamination of ethane as identified in the monthly ground water samples from 1998 to 2012.   Client:  The client is a leading environmental management consulting firm in USA.     Geography: USA   Industry: Environment   Challenges: · Client was facing shortage of technically skilled man power in high end 3D GIS model creation. · Client was in urgent need of these services so that expecting a quick turnaround time. Approach: SBL’s 3D modeling team has plotted the well locations from the input data base, initially in 2D and later converted to 3D.  During this conversion the team has used the well screen height with respect to MSL as the z-element.  In the next phase of the project the ethane concentration in µgm/liter was plotted on a 3D surface.  Later 3D contours with various intervals, based on the concentrations of the ethane were created.  This analysis was repeated month wise to get the time series analysis of the ethane concentration.   Benefits: Client benefits included: · The work was done well within the schedule and budget of the client. · Client could able to demonstrate that ethane concentrations has reduced spatially and depth wise. · Time series analysis on a 3D environment was possible with the generated data. · Client could able to demonstrate that ethane concentrations was reduced and contained by 2012 http://www.sblcorp.com/GIS_Case_Studies.php

Critical point detection along power transmission lines using LIDAR data

The LIDAR data turns to be the most accurate terrain depicter in the recent past for the utility corridor mapping. Power transmission lines are the most difficult utility to monitor due to 

its aerial existence and thin wires.Critical point detection along the immediate corridor is a necessary pre requisite for the maintenance of the power transmission lines. Height information about the objects in the immediate vicinity, especially vegetation is key factor for a maintenance team. The LIDARdata in .las file format can be classified for ground and non ground clouds using automatic macros. Quality checks will refine quality of ground points. Among the non ground points power lines strings can be captured using top views and need to ensure that it connects all the towers. This classification is the result of analysis by an automated filter developed which detects all LIDAR hits returns from the power lines and can be classed as wires. Towers were also can be detected in a similar way.Critical points can be detected along the transmission lines and exact height needs to be assigned to each object. 50m either side of the transmission lines can be considered for object search and exact height of the objects can be derived.The objects encountered will be houses and vegetation in general. A vegetation clearance report will help to details location of the critical points and its complete information. 

SBL Geo spatial services, the Geo Spatial wing of SBL, an ISO accredited global ITES Company based in Cochin, India, weaves out its feature mapping solutions with accuracy and quality which no one else offers. The team is a pack of expert technocrats who make use of Remotly Sensed data on the information on the surficial relief and spatial distribution of the features to map the terrain accurately.

http://www.sblcorp.com/Geospatial-Services.php

High Accuracy DTM Editing and Orthophoto of a Mining Area Using UAV Imagery

Abstract: This project was executed by SBL for a mining company having operations in Africa. The study area covered approximately 32 sq.km of land falling in the south western part of Congo.This project was awarded to SBL based on the excellent quality and timeliness shown during an earlier project for Landuse and Landcover mapping. The client requirement was for the creation of hight accuracy DTM edited to 0.2m, DSM of 0.2m and orthophotos of 10cm GSD using UAV imagery

Client: Client is a mining company based in the UK and having mining operations in various parts of African continent

Geography: ~32 sq.km of land falling in the south western part of Congo
Industry: Mining .

Challenges:
• GCP distribution was not even and so getting a good quality stereo model became difficult. So SBL’s experts suggested suitable areas for collecting additional GCP’s which proved to be very useful in the end.
• Fixing the terrain height and water body was a challenge as the area is a dense forest with large swathes of swampy land.
• Data being of 0.2m handling and viewing it was difficult.
• Editing 0.2m DTM was a very big challenge.

Approach: SBL used state of the art UAV softwares in carrying out the Aerial Triangulation of the UAV images. After Point cloud densification and DSM creation Undistorted images were generated by the UAV software after the creation of DSM data. The Aerial Triangulation reports were imported into Inpho Match AT and stereo models were created in it. After this DTM were generated and edited using Inpho DT Master software. SBL’s highly experienced professionals produced high accuracy DTM by the correct usage of soft and hard breaklines . All manmade structures and vegetated areas were interpolated to terrain height after plotting breaklines which were fixed to terrain heights. The edited DTM points were exported in XYZ format and edge matched with adjacent DTM tiles and dispatched to the client.


Benefits: The client benefited by this work in
• Getting high accuracy DTM’s of 0.2m by the use of cutting edge UAV softwares.
• Very quick turnaround time.
• Use of photogrammetry software along with UAV software helped in getting a good DTM output.

http://www.sblcorp.com/services/geospatial-services/

Light Detection and Ranging (LiDAR) systems

A visual remote sensing technology used to collect a wide range of topographic data Lidar services (Light Detection And Ranging ) is an contraction for Light Detection And Ranging, sometimes also referred to as Laser Altimetry or Airborne Laser Terrain Mapping (ALTM). The LiDAR system basically consists of addition of three technologies, namely, Inertial Navigation System (INS), LASER, and GPS.The Global Positioning System (GPS) has been fully operational for over a decade, and during this period, the technology has proved its potential in various application areas. Some of the important applications of GPS are crustal deformation studies, vehicle guidance systems, and more recently, in LiDAR.Lidar Data Mapping also known as airborne laser scanning (ALS), is an emerging remote sensing technology with promising potential to assisting mapping, monitoring, and assessment of forest resources. Compared to traditional analog or digital passive optical remote sensing, LIDAR offers tangible advantages, including nearly perfect registration of spatially distributed data and the ability to penetrate the vertical profile of a forest canopy and quantify its structure.

The LiDAR system includes parameters that let the point distribution on the ground to be adjusted. Normally, the parameters are placed to generate a uniform distribution of points for the most applications. The LiDAR can also distinguish intensities of the return, which can then be used to develop a set of data points that resembles a black and white photo. The intensity data can be laid over the height elevation model. Some triangulating of points are performed by the software to give us a quick view of what the processed data looks like, which is why the trees appear as triangles. There are several major components to a LIDAR system Laser, Scanner, Photodetector, Position and navigation systems

Laser scanning technology is changing many aspects of surveying profession today. Possibility to capture huge amount of georeferenced data in a short period of time is raising challenges both to hardware and software. Many industrial facilities lack modern digital documentation needed for effective maintenance an upgrading. Old paper maps and blueprints are usually two-dimensional and based on classical surveying methods.

The bare earth digital elevation models (DEM) are a representation of the earth's surface where all man-made structures and vegetation have been removed.The bare earth DEMs were created from a subset of LiDAR returns that were classified as ground.They are regularly gridded at six-foot post-spacing and were derived using TIN processing of the ground point returns. The elevation values are in feet.The DEM data format is ArcInfo interchange. Terra Point surveyed and created this data for the Puget Sound LiDAR data processing Consortium under contract.

http://www.sblcorp.com/services/geospatial-services/lidar-services/