Topocad 23

Explanations for Observations

An explanation of the columns follows under the Observations tab:

From Point
Select from which point you have made the observation, i.e. the station point. This may be both a known point and a free station, or a new point in the centre of the traverse.

To point
Marks the point to which the measurement is made.  This could be both a known or a new point.

Series no.
Normally you measure one direction series at a time per station and then change the station point. If you have measured in this way, you do not need to worry about this column which will then have a default value of 1 for all observations. However, if a special case occurs where you measure one more direction series from the same station straight after the first series, the series need to be separated from each other in some way. If this does not happen, the program treats both series as one which may lead to errors. We differentiate between the series by manually assigning the value of 2 in the series column to the other direction series. If we have a third series from the same station immediately after the second we assign these observations the value of 3 etc... If several station establishments occur in a row from the same point in a survey data file, the net adjustment when importing will set different series numbers automatically to separate the measurement series.

Hor. angle
Horizontal angle.

Vert. angle
Vertical angle.

Length
Slope distance. If the vertical angle field on the same row is blank, the length is treated as horizontal.

Height diff.
Measure the height difference between the from and to point. Used primarily for leveling data.

Bearing
Here you can enter a known bearing between two points. It could either be a fictitious bearing to give the net the desired orientation (turned facing north), or a bearing measured using gyrotheodolite.

Instr. elevation
Height of instrument above the point.

Refl. height
Reflector (prism) height above the point.

Instruments
Specify the instrument used, which in turn defines the precision of the observations (measured as accuracy), which is displayed under the instrument tab.

Proj. corr
Projection correction - specifies if this is to be used or not for the observation. Speed settings are available in Settings (see this chapter for a more detailed description) if you have selected Use project settings, which generally activates/deactivates this function for all observations regardless of what has been specified for each individual observation. The projection correction formulas used are described in HMK Geodesi Stommätning Chap. C2.

Ellips. corr
Ellipsoid correction - specifies if this is to be used or not for the observation. The correction reduces measured lengths to the ellipsoid. The height correction formulas used are described in HMK Geodesi Stommätning Chap. C1. Just as for the projection correction, the speed settings will take precedent over the individual settings for an observation.

Atm. corr.
Atmosphere correction to lengths. This function is affected in the same way as the projection correction to the speed settings in Settings. The corrections are calculated as follows (obtained from instrument manuals from the manufacturer in question):

Leica
ppm=281.5-((0.29035* pressure)/(1+0.00366* temp))

Trimble/Geodimeter
ppm=275-((79.53*pressure)/(273+temp))

Topcon
ppm=279.6-((79.53*pressure)/(273.2+temp))

Sokkia Laser
ppm=282.59-((0.2942*pressure)/(1+0.003661*temp))

Sokkia Reflector
ppm=278.96-((0.2904*pressure)/(1+0.003661*temp))

Pressure and temperature are specified as mbar and degrees. The lengths are then corrected by multiplying by the ppm figure. If the length is specified in km, the correction is given in mm.

Pressure
Atmospheric pressure. Consideration is taken to this only if Yes had been entered in the Atm. corr. column. If you have the values in mmhg you recalculate them to mbar by multiplying by 1.3333, which is simply done using the Search/Modify function that you activate by right-clicking.

Temp
Temperature in degrees. Consideration is taken to this only if Yes has been entered in the Atm. corr.

Weight f. length
Weight factor length. Weights for lengths are automatically calculated through the formula P= 1 / mf2, where mf is the observation's mean error that is obtained from the instrument data. This value does not need to be changed by the user. If you end up in a situation where you know that an observation is worse than expected due to external circumstances (e.g. weather, light conditions, instrument errors), or if you, for whatever reason, would like certain observations to have less of an effect on the results, you can reduce the weighting of the observation. For lengths, this is done by changing the weight factor from 1 (=unaffected) to a lower value. If we change to 0.5, for example, this particular length will affect the result half as much as normal (the previously calculated weight is halved).

Weight f. angle
Weight factor angle. See above for explanation.

Weight f. height
Weight factor height. See above for explanation. Apart from levelled heights, this can also be used for an observation of the vertical angle and length if trigonometric heights are to be used. Weights for heights are calculated for leveling automatically using the formula P= k / L where L is the length between the points in km. k is a constant that is set to one if only one instrument is used. If several instruments have been used, k is set for the observations with the best instrument to one and for the others to one divided by how many times worse the observation's instrument is compared to the best instrument (calculated from the instruments' apriori mean errors).

Use observation
This tab has a number of selections and all of them specify the observations for the current row to be included in the calculations: