page_title: Temporary adjustments of theodolites
section_title: Levelling
hierarchical_section_title: Temporary adjustments of theodolites / Levelling
caption_attribution_description: English: A bull's eye level with green backdrop, with the centered bubble showing a level position.
context_page_description: Temporary adjustments are a set of operations which are performed on a theodolite to make it ready for taking observations. These include its initial setting up on a tripod or other stand, centering, levelling up and focusing of eyepiece.
context_section_description: Leveling of an instrument is done to make it vertical axis with respect to the apparent force of gravity at the station.
  For two spirit vials at right angles:
  Bring one of the level tubes parallel to any two of the foot screws, by rotating the upper part of the instrument.
  The bubble is brought to the centre of the level tube by rotating both the foot screws either inward or outward. The bubble moves in the same direction as the left thumb.
  The bubble of the other level tube is then brought to the centre of the level tube by rotating the third foot screw either inward or outward. [In step 1 itself, the other plate level will be parallel to the line joining the third foot screw and the centre of the line joining the previous two foot screws.]
  Repeat step 2 and step 3 in the same quadrant till both the bubble remain central.
  By rotating the upper part of the instrument through 180°, the level tube is brought parallel to first two-foot screws in reverse order. The bubble will remain in the centre if the instrument is in permanent adjustment.Otherwise, repeat the whole process starting from step 1 to step 5.
  The same principle applies for a bulls-eye level:
  Bring the level parallel to any two of the foot screws, by rotating the upper part of the instrument.
  The bubble is brought to the centre of the level tube by rotating both the foot screws either inward or outward.
  Rotate the upper part of the instrument through 180°, so the level is over the remaining foot screw. The bubble will remain in the centre if the instrument is in permanent adjustment. If not adjust this screw to halve the error. Then rotate back through 180° and check the error. Adjust those screws to halve the residual error.  Continue until the bubble is always central on the ring.
caption: Bull's eye level with alignment rings
exif: {"Image ImageDescription": "SONY DSC", "Image Make": "SONY", "Image Model": "DSLR-A100", "Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "DSLR-A100 v1.04", "Image DateTime": "2009:06:08 23:01:03", "Image YCbCrPositioning": "Centered", "Image ExifOffset": "390", "Image PrintIM": "[80, 114, 105, 110, 116, 73, 77, 0, 48, 51, 48, 48, 0, 0, 0, 6, 0, 1, 0, 22, ... ]", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail Orientation": "Horizontal (normal)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "40294", "Thumbnail JPEGInterchangeFormatLength": "3138", "Thumbnail YCbCrPositioning": "Centered", "EXIF ExposureTime": "1/4", "EXIF FNumber": "5", "EXIF ExposureProgram": "Aperture Priority", "EXIF ISOSpeedRatings": "200", "EXIF ExifVersion": "0221", "EXIF DateTimeOriginal": "2009:06:08 23:01:03", "EXIF DateTimeDigitized": "2009:06:08 23:01:03", "EXIF ComponentsConfiguration": "YCbCr", "EXIF CompressedBitsPerPixel": "8", "EXIF BrightnessValue": "3/4", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "3", "EXIF MeteringMode": "CenterWeightedAverage", "EXIF LightSource": "Unknown", "EXIF Flash": "Flash did not fire, compulsory flash mode", "EXIF FocalLength": "100", "EXIF SubjectArea": "[1936, 1296, 3872, 2592]", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "3872", "EXIF ExifImageLength": "2592", "Interoperability InteroperabilityIndex": "R98", "Interoperability InteroperabilityVersion": "[48, 49, 48, 48]", "EXIF InteroperabilityOffset": "25212", "EXIF FileSource": "Digital Camera", "EXIF SceneType": "Directly Photographed", "EXIF CustomRendered": "Normal", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF DigitalZoomRatio": "0", "EXIF FocalLengthIn35mmFilm": "150", "EXIF SceneCaptureType": "Standard", "EXIF GainControl": "None", "EXIF Contrast": "Normal", "EXIF Saturation": "Normal", "EXIF Sharpness": "Normal"}