Lesson Note On Leica Total Station – Surveying from a Known Point

Leica Total Station – Surveying from a Known Point

When the total station is set up and level over the known point it will require another known point to help calculate the coordinate reference system that the unknown measurements will be measured with. That is to say that the total station will be able to know the angular relation between the easting, northing and elevation of itself and the Easting northing and elevation of the second known point (often referred to as the back sight). Then all of the angles and distances measurements calculated and computed with the laser will be translated into the same east, northing and elevation coordinate space as well.

Create a Measure Job File

From the main menu, select Meas job management by pressing the number 1 on the keypad (from the Main Menu).  The measure job created will be the file that your measured points in your survey will be recorded

.

Next press F2 to open the Create new job menu.

Leave all settings at default and enter a new job nameby using the letters on the keypad.

The F6 key (NUM) will switch the key pad between numbers and letters.  Note it may take some practice to get used to this switching back and forth.

After the new file name has been entered press F1 to continue.

Next ensure that the job file that you have just created is highlighted and select CONT again by selecting F1from the keypad.

Create a Data Job File

The data job is where the known or fixed points are stored (the Data job file can be the same as the measure job file but often it is good survey practise to keep these two files separate).

Steps followed for creating a data job file are very similar to those mentioned above for creating a measure job.

From the main menu, select data job management by pressing the number 2 on the keypad.

Next press F2 to open the Create new job menu (and provide a file name like you did above) or select an existing data job file if one exists. Then press F1 to continue.

If it is a new data job file then you will need to input values for the known points.

Select the FNC key from the bottom grey keypad.

Select menu item 5 Data view and Edit

On the next screen select Input from the function key options by pressing the F3 key.

Enter the name of the point id of the known point you wish to create.

Enter the known Easting, Northing and Elevation values for this point.

Press REC to accept the value of the point id

Repeat the steps to enter all the known points that you wish to use in your survey and then use the ESC key until you reach the main menu again.

Select your Codelist

From the main menu, select codelist management by pressing the number 3 on the keypad.

Select the name of the code list you created earlier with the Leica Survey Office software and uploaded to the memory card. Then press F1 to continue.

Record Backsight Locations

From the Main Menu select Setup from the bottom of the screen by pressing F5.

In the Job settings menu you should see the name of your measure job, data job, and code list. If either looks incorrect than go back and select them again using the instructions from the start of this manual.

Select QSET (quickset) from the bottom of the menu using F4.

For the Station Id enter the point id of the known point for the surveying monument that you have the total station set up upon.

Next enter the Backs. Id to select the backsight point id

Enter the instrument height of the total station that you recorded previously in the initial setup.

Enter the base height of the reflector poles used to collect the various points during the survey.  The number to enter here will be the value found above the grasp of the reflector pole. A height of 2 meters is common height for reflector poles in most surveys but sometimes there is a need to use different height values.

Aim the total station instrument towards the survey prism with the and press F2 on the key pad which selects DIST. Ensure that the range pole is vertical and plumb by centering the bubble on the pole. This will allow the Total Station to compute the delta of what you told it where it was and where it is based on the reading of shooting the pole.

Take note of the delta horizontal distance on the screen. Anything under 2cm is considered an acceptable value. Once accuracy under 2cm is achieved hit CONT by pressing F4 on the key pad.  The unit will now know spatially where it is located and surveying of the unknown points can commence.

The screen should now have an option MEAS appear in the lower right corner of the screen, this is an indication that you can now shoot to any unknown point with the reflector. Press the F6 function key to enter into the measure and Record menu. Here you can point and aim at the reflector, enter the point Id value and then press theF3 function key to record the coordinate values of that point. The total station will increment the point ids taken automatically, or you can change the values manually each time.

Tip: Remember to adjust the height of reflector on this screen if the height of the reflector unit is adjusted during the survey.

To finishing surveying simply press ESC until the screen is back to the main menu. Press both the ON button and the left arrow button at the same time to shut down the unit.

Tip: How to adjust the view of the Total Station
Use the “sight” on the total station to roughly point the Total station at the reflector.  Then use the knobs to fine tune the view at the reflector target.  Use the focus on the lens to ensure a clear and focused view of the target and that the cross hairs are centered on the center of the target.

SOLUTION ON PHOTOGRAMMETRY PROBLEMS PART 1

Examples

1) The scale of an aerial photograph is 1 cm = 100 m. The photograph size is 20cm x 20cm. Determine the number of photographs required to cover an area of 100 sq. km if the longitudinal lap is 60% and the side lap is 30%.

Solution.

Here                l = 20 cm  ;  w = 20 cm  ;  P₁ = 0.60 ; P_w = 0.30

                         

 The actual ground length covered by each photograph is

                        L = (1 – P_l) sl = (1- 0.6) 100 x 20 = 800 m = 0.8 km

Actual ground width covered by each photograph is

                        W = (1 – P_w) sw = (1 – 0.3) 100 x 20 = 1400 m = 1.4 km

 Net ground area covered by each photograph is

                        a = L x W = 0.8 x 1.4 = 1.12 sq. km

hence number of photographs required is

                       

2) The scale of an aerial photograph is 1cm = 100 m. The photograph size is 20cm x 20xm. Determine the number of photograph required to cover an area 10 km x 10km, if the longitudinal lap is 60% and the side lap is 30%.

Solution

Here                L₁ = 10 km  ; L₂ = 10 km

  Number of photographs in each strip is given by

           

Number of flight lines required is given by

           

Hence number of photographs required will be           

The spacing of the flight lines would be 10/9 = 1.11 km and not 1.4 as calculated theoretically in the previous example.

LESSON NOTE ON USES OF PHOTOGRAMMETRY

USES OF PHOTOGRAMMETRY.

Photogrammetry can be used for the following purposes.

(1)         Preparation of image maps useful for many reconnaissance and planning purposes. Such image maps include digital mosaics (unrectified, rectified, controlled, uncontrolled)..

(2)         Compilation of planimetric and topographical maps and plans of large areas.

(3)         Preparation of route maps for roads, power-lines, pipelines, canals for storm waters drainage.

(4)         Computation of positions, distances, areas of geometric objects, volumes.

(5)         Generation of DTM (DEM) and perspectives (drapped maps)

(6)         Production of orthophoto map

(7)         Mapping of inaccessible areas

(8)         Freezing of a rapidly changing phenomenon for study

(9)         Documentation of Archeological monuments.

Photogrammetry is rapid and economical for the survey of large areas than any other method.

LESSON ON Procedures of Photogrammetric Mapping

Procedures of Photogrammetric Mapping

The procedures in photogrammetric mapping include project planning (recce), flight planning (or image acquisition planning), imaging, ground control (ground truth) establishment, image restitution (interior, relative, absolute and exterior orientations), QA/QC analysis, collection of vector data and final map production.

 1 FLIGHT PLANNING FOR AERIAL IMAGE COVERAGE

PHOTOGRAMMETRIC PROCEDURES

As presented in the block schematic above, photogrammetry, the science of measurement by aerial photographs, consists of five stages which are summarized below:

Site Identification and flights/image-acquisition planning. Aerial surveying is generally carried out over large areas which involve extensive photography or image coverage. Such surveys are, therefore, made by the government, organizations or large private companies. In general, prior to the acquisition of the images, the site must be identified, possibly using existing maps or mosaics. All the possible ways of covering the area with images are considered and the most appropriate method selected. For aerial photography, the imaging will usually be done in strips and so the process(of image acquisition) will involve the determination of the number of images per strip and the number of strips to cover the area. Of course, flight planning makes use of the photographic parameters such as focal length, image frame size, flying height and desired ground sampling distance (gsd) and the amount of overlaps needed both between successive images and between strips. Consideration for photo scale is also made. The photo scale is directly linked with the accuracy of the map which is the final product. After agreeing on the map accuracy with the client, the scale of the aerial photography is then determined. For topographical maps of small and medium scales, the height accuracy attainable and the image quality are the governing factors for the choice of the scale, whereas for large scale maps, planimetric accuracy is more important. The largest scale of maps that can be produced photogrammetrically in an economic way is 1: 500. It is therefore essential to ensure that both planimetric and height accuracy requirements, are met with the decided photo scale. The flying height of the camera depends on the scale of the photography. For simple plotting, the average scale of the photography is generally kept the same or slightly larger than the desired scale of the final compiled map. Hence, by knowing the average scale of the photography and the scale of the compiled map, the flying height of the aircraft above mean ground level, may be calculated. If planimetric accuracy is important, the flying height depends on the photo scale chosen to meet the planimetric accuracy. On the other hand, if height accuracy is of prime importance, the flying height may be first decided, depending on the required contour interval. The flying height is often related to the contour interval of the final map.

The following points may be noted :

(i) The area of ground covered by each photograph increases with the increase of the flying height of the aircraft and hence less number of photographs are required for any particular area of land.

(ii) The scale of the photography increases with the decrease in the flying height and consequently more details for greater height accuracy are obtained.

(iii)Due to increased flying height, the haze and dust reduce the quality of the photographs.

(iv)The cost of the flying at greater flying height is excessive as compared to that of low flying height.

The details of this process are given later. In the case of satellite imagery, the total number of frames to cover the area is calculated from the available frame size, and the amount of overlaps that exist on the images. In some cases, only one frame or two may be sufficient for the purpose when the frame is large.

LESSON ON PHOTOGRAMMETRIC PROCEDURES PART 3

NUMBER OF PHOTOGRAPHS NECESSARY TO COVER A GIVEN AREA

In the preliminary estimate, the number of photographs required is calculated by dividing the total area to be photographed by the net area covered by a single photograph.

Let       A = total area to be photographed.

            l = length of the photograph in the direction of flight

            w = width of the photograph normal to the direction of flight

            s = scale of photograph =  (i.e., 1 cm = s metres)

            L = net ground distance corresponding to l

            W = net ground distance corresponding to w

            a = net ground area covered by each photograph = L x W

P_t = percentage overlap between successive photographs in the direction of flight (expressed as a ratio)

P_w = side lap (expressed as a ratio).

Since each photograph has a longitudinal lap of  the actual ground length (L) covered by each photograph is given by

                                                                                                  (43)

Similarly, the actual ground with (W) covered by each photograph is given by                                                                                                     (44)

Hence the ground area (a) covered by each photograph

                            (45)

The number of the photographs (N) required is given by      

N = A/a                                                                        (46)

If, however, instead of the total area A, the rectangular dimensions (i.e., length and width) of the ground are given, the number of the photographs required are computed by calculating the number of strips and the number of photographs required in each strip and multiplying the two.

Let       L₁ = dimension of the area parallel to the direction of flight

            L₂ = dimension of the area normal to the direction of flight

            N₁ = number of photographs in each strip

            N₂ = number of strips required

            N = total number of photographs to cover the whole area.

Now net length covered by each photograph = L = (1 – P_l) sl

  Number of photographs in each strip is given by

                                                                 (47)

Similarly, net width covered by each photograph = W = ( 1 – P_w) sw

Hence the number of the strips required are given by

                                                              (48)

Thus, the number of photographs required is

                          (49)

Lesson Note On Principles of Town planning

Principles of Town Planning

Town planning cannot be studied in isolation. It involves the study of various subjects such as engineering, architecture, surveying, transportation planning etc. The intention of the town planning is to satisfy the needs of our future generations and prevent the haphazard growth of the town. Some of the guiding principles of town planning are as follows:

1. Zoning

The town should be divided into suitable zones such as commercial zone, industrial zone, residential zone, etc and suitable rules and regulations should be formed for the development of each zone.

2. Green belt

Green belt is non-development zone on the periphery of the town. It prevents the haphazard sprawl of the town restricting its size. In essence, a green belt is an invisible line designating a border around a certain area, preventing development of the area and allowing wildlife to return and be established. Greenways and green wedges have a linear character and may run across the town and not around the town.

3. Housing

Housing has to be carefully studied and designed to suit the local population. Care should be taken to see that there is no development of slums since it would be responsible for degrading the life of the citizens. There are various types of housing styles. When a landuse plan is made, zones for independent housing, midrise buildings, high rise buildings are allocated. Landuse maps are of two types. Type 1 helps us study the landuse on a broad range. All we can see are the residential, commercial and recreational zones.

4. Public buildings

Public buildings should be well grouped and distributed throughout the town. Unnecessary concentration of public buildings should be avoided. Factors such as parking facilities, road widths have to be taken into consideration while allocating the space for public buildings.

5. Recreation centres

Recreation centres have to be given importance while designing a town. They are necessary for the recreational activities of the general public. They include parks for walking and cycling, amusement parks etc.

6. Road systems

Road network hierarchy is very important. The efficiency of any town is measured by the layout of its roads. A nicely designed road system puts a great impression in the minds of people, especially the visitors to the town. The provision of a faulty road system in the initial stages of town formation proves to be too difficult and costly to repair or to re-arrange in future.

7. Transport facilities

The town should be provided with suitable transport facilities so that there is minimum loss of time from place of work to the place of residence. Efficiency in transport facilities includes both public and private networks. Public transportation network includes access to buses, trains, trams and trolleybuses. Efficiency in using the public transport will determine the success of that town in terms of design.

Town planning has gained a lot of importance today. New towns are being developed. It has become very important for the town planners to concentrate on old development as well as the new development. It is essential that old and new development are linked properly. Energy efficiency in planning should be the goal of any town planner, urban designer or an Architect.