Friday, 10 May 2019

Create your own Shapefile in Arcgis

Create your own Shapefile in Arcgis
If you wish to map certain points of interest, routes, or polygons but cannot find a shapefile, you can create your own.  This tutorial explains the steps involved in creating and editing a shapefile.

Step 1:  Creating your Shapefile
Open ArcCatalog
Navigate to your project folder
From the menu, click File>New>Shapefile
Give your shapefile a name and select a geometry based on what type of features you are trying to draw
Use the Edit button to select the coordinate system that you are using in your ArcMap project
To have a full details on self training visit www.heroizutech.com.ng and request a copy of A-Z of Arcgis 

Step 2: Editing your Shapefile
Add your shapefile to your ArcMap project
Open the Attribute Table of your shapefile
 Using the Attribute Table Options button, use the Add Field button to add fields you want to keep track of.  For example, if plotting metro stations, you may want to add a field for the name or number of the stop.
Optional: Set the symbology of your fields now.  This will save time later when creating features.
Turn on the Editor toolbar by going to Customize>Toolbars>Editor
Click on the Editor button and push Start Editing
Make sure your shapefile is selected in the Create Features window on the right
The bottom of this window shows the different construction methods available for your shapefile.  Click on one to start drawing.  You’ll notice that your cursor turns into a cross.
Click on your map to place a vertex.  You can place multiple vertexes to trace a route on your map.
https://civli-engineering.blogspot.com/2019/06/create-drawing-template-in-autocad.html

When finished drawing, right click and select Finish Sketch, alternatively you can push F2
To edit the attribute table for the feature you just created push the Attributes icon on the Editor toolbar.
All the editable fields for the feature you just created will appear.  This way you can label your features as you create them
When you’re done creating features click on the Editor button on the Editor toolbar and select Save Edits, and then Stop Editing
The symbology of the shapefile you just created can be manipulated like any other shapefile

Tips
 The snapping tool allows you to create features more accurately.  It will cause your cursor to snap to vertices, edges, and other features that you have created.  This will allow you to accurately place points on routes that you’ve created, or create polygons that do not overlap or have gaps between them.  To turn it on go to Customize>Toolbars>Snapping and turn the tool on in the toolbar that appears.
You can only edit the values in the Attribute Table of your shapefile while in Editing is turned on.
However, you cannot create new fields in the Attribute Table while Editing is turned on.  Create fields before starting an editing session.
If you set your symbology for your shapefile before you start the Edit session ArcMap will automatically create templates for your construction tool.

How to Converting a .jpg map into a shapefile In Arcgis

Converting a .jpg map into a shapefile

First 

There is no direct route to convert an image into a shapefile format. Your jpg map has no spatial reference. You can load it into arcmap but it won't know where to put it. In order to tell arcmap where it belongs in space you have to provide geographic reference points, hence the term 'georeferencing'.

In ArcGIS this is done via the Georeferencing Toolbar, which is turned off by default. To turn it on you just right-click on a toolbar, look for Georeferencing and turn it on. Then you will use the 'Add control points' tool (first button after right of the combo box) to tell arcgis what points on the image correspond to points on a second data source that already has a spatial reference.

Here are the steps I recommend following:

  1. Open a new arcmap document.
  2. Load an existing, vector shapefile of the counties you are interested in
  3. Load the jpg of the map you want to georeference
  4. In the Table of Contents, right click on the map and select 'Zoom to Layer'
  5. Click on the 'Add Control Points' button on the georeferencing toolbar
  6. Find a point on the map that you can match to the shapefile you loaded in step 2 and click on it. I recommend finding places that are easy to identify such as intersections of major roads, sharp corners in boundaries, etc
  7. Then click on the 'Previous Extent' arrow on the toolbar to zoom back to your shapefile and move the map so that you can see the point you picked in the previous step in the shapefile.
  8. Click on the corresponding point in the shapefile. You should see your map appear in the vicinity of where you clicked. It may be very small, or it might be huge, don't worry, you'll fix that with the next step.
  9. Now you are going to repeat the last two steps using a second point on the jpg map, preferably one that is relatively far from the first one, but just go with what you can for now.
  10. Repeat the process a few times and try to get the jpg as closely matched to the shapefile as possible. You'll never get it 100 percent accurate, but you can get pretty close if you try. If something goes crazy and the map gets really distorted you can either start over entirely or delete the last control point you added using the 'View Link table' button on the georeferencing toolbar.

Then it's just a matter of either editing your existing shapefile or creating a new one using the jpg map as a background. You'll probably have to assign some attribute data such as "Damaged" or "County Name" to the polygons you draw but that is a whole other issue.

I'm sure it sounds complicated if you've never done it before but it's really pretty easy once you get your head around the concept. Just remember that there are three fundamental steps here 1) Georeference the jpg so your GIS knows where it is in space, 2) Create a new dataset from the jpg in vector format, 3) Use the attribute table from your new dataset to conduct your analysis.

Here are some links that may be helpful: Get A - Z of Arcgis text book from HeroizuTech Services Ltd

http://www.heroizutech.com.ng 

For the second phase you will have to create a new shapefile. You will use the editing tools inside arcmap to digitize the affected areas as new polygons.

1) Create a new shapefile. Open up arcCatalog and navigate to the folder you are working in. Right click and select New --> Shapefile...

2) Give your shapefile a name and under feature type select 'Polygon'

3) Click the Edit... button in the lower right and select the coordinate system you want to work with, this should probably be the same as the one used by the county base file you used to georef the map jpg.

4) Open your arcmap document with the georeferenced jpg map and add your new shapefile to the document.

5) Right click on your new shapefile in the Table of Contents, got to 'Edit Features' and select 'Start Editing'.

6) Now you can use the 'Create Features' window to digitize the affected areas on your jpg map. Make sure to save your edits often using the Editing Toolbar.

7) Once you have drawn/digitized over all the affected areas save your edits and click on the 'stop editing' button.

8) Right click on your shapefile in the table of contents and select 'Open Attribute Table'. In the window that pops up, click on the drop-down button in the upper left and select "Add Field"

9) Name your field 'Area' and select 'Double' under type.

10) You should see your new field show up as a column on the right of your attribute table, right click on its heading and select 'Calculate Geometry', choose area and the units you want, this will tell you the area of each of the polygons you digitized.

That should give you everything you need to show the affected areas on a map and calculated the total area. Again, if you need further details or more instructions these steps have been heavily documented all over the web and a search for 'arcgis create shapefile' or something similar will give you lots of results with pictures/video that will be more descriptive than my summary.


Regarding your last comment: You should now have three items in your table of contents. One should be the jpg map you georeferenced, a second should be the county map you used to georeference the jpg map, and the third should be the polygons of affected areas you just digitized.

To make a map that you can export and use as an image or figure somewhere else (ie a report or webpage) you will need to switch ArcMap over to 'Layout' view. (Go to the 'View' menu and choose 'Layout View') This is where you can add items like a legend, north arrow, and scale (via the Insert menu). Once you have things looking the way you would like them go to File, Export Map..., and choose the location, format, and resolution for the file you want to export. Then you will have an image file of the map you just laid out that you can use elsewhere.

I realize those instructions are pretty bare bones - if you need more help than that then that should really be a separate question, or the focus on how to get my book A-Z of Arcgis 

Thursday, 2 May 2019

GIS Tutorial 1 Lecture 6 Digitizing. - ppt download

GIS Tutorial 1 Lecture 6 Digitizing. - ppt download: Outline Digitizing overview GIS features Digitizing features Advanced digitizing tools Spatial adjustments Digitizing sources GIS TUTORIAL 1 - Basic Workbook

Sunday, 11 November 2018

QUESTION: Explain grid levelling method

     Explain grid levelling method
 Image result for Explain grid levelling method

                     In this method, the area to be surveyed is divided into a grid or series of squares. The grid size may vary from 5 m x 5 m to 25 m x 25 m depending upon the nature of the terrain, the contour interval required and the scale of the map desired. 
Image result for Explain grid levelling method
Also, the grids may not be of the same size throughout but may vary depending upon the requirement and field conditions. The grid corners are marked on the ground and spot levels of these comers are determined by leveling. 
Image result for Explain grid levelling method
The grid is plotted to the scale of the map and the spot levels of the grid corners are entered. The contours of desired values are then located by interpolation. 
Image result for Explain grid levelling method
Special care should be taken to give the spot levels to the salient features of the ground such as hilltops, deepest points of the depressions, and their measurements from respective corners of the grids, for correct depiction of the features. The method is used for large scale mapping and at average precision. 

QUESTION: DESCRIBE THE SOURCES OF ERROR IN LEVELING


            DESCRIBE THE SOURCES OF ERROR IN LEVELING
Image result for reciprocal levelling

            Many sources of error exist in levelling and the most commonly met in practice are discussed. Firstly, one of the sources of error is errors in the equipment which is collimation error. This can be a serious source of error in levelling if the sight lengths from one instrument position are not equal, since the collimation error proportional to the difference in sight length. The line of collimation should be parallel to the line of sights. 
Image result for reciprocal levelling
Hence, in all types of levelling, sights should kept equal, particularly back sights and fore sights. Before using any level it is advisable to carry out a two-peg to ensure that the collimation error is as small as possible. Other than that, compensator not working. The function of compensator is to deviate the horizontal ray of light at the optical center of the object lens through the center of the cross hairs. This ensure that line of sight viewed through the telescope is horizontal.  
Image result for reciprocal levelling
If the reading changes to a different position each time the footscrew is moved or thr instrument tapped, the compensator is not working properly and the instrument should be returned to the manufacturer for repair. Parallax  also one of error in the equipment. Parallax must be eliminated before any readings are taken. Parallax is occur when the image of the distance point or object and focal plane are not fall exactly in the plane of the diaphragm. 
Image result for reciprocal levelling
To eliminate parallax, the eyepiece is first adjusted until the cross hairs appear in sharp focus. Then, defects on the staff  which is the incorrect graduation staff cause the zero error. This does not effect height differences if the same staff is used for all the levelling but introduces errors if to staves used for the same series of levels. When using a multisection staff, it is important to unsure that it is properly extended by examining the graduations on either side of each joint. The stability of tripods should also be checked before any fieldwork commences .
Image result for reciprocal levelling 
                     Secondly, field errors also source of error. The example of field errors is staff not vertical, failure to hold the staff vertical will result in incorrect readings. The staff is held vertical with the aid of a circular bubble. At frequent intervals the circular bubble should checked against plumb line and adjusted if necessary. Another example of field errors is unstable ground. When the instrument is set up on soft ground and bituminous surfaces on hot days, an effect often overlooked is that the tripod legs may sink into the ground or rise slightly while readings are being taken.This alters the height collimation and therefore advisable to choose firm ground on which to set up the level. 

After that, handling the instrument and tripod as well as vertical displacement, the HPC may be altered for any set-up if the tripod is held or leant against. When levelling, avoid contact with the tripod and only use the level by light contact through the fingertips. Then, instrument not level is also the field errors. For automatic levels this source of error is unusual but, for tilting level in which the tilting screw has to be adjusted for each reading, this is common mistake. The best solution is to ensure the main bubble is centralised before and after reading.
 Image result for reciprocal levelling
                 Thirdly, source of error is the effects of curvature and refraction on levelling. The effect of atmospheric on the line of sight is to bend it towards the Earth’s surface causing staff readings to be too low. This is variable effect depending on atmospheric condition but for ordinary work refraction is assumed to have value 1/7 that of curvature bit is of opposite sign. The combined and refraction correction is c + r = 0.0673 D². If longer sight lengths must be used, it is worth remembering that the effects of curvature and refraction will cancel if the sight length are equal. But, curvature and refraction cannot always be ignored when calculating heights using theodolite methods.
 Image result for reciprocal levelling
                  Lastly, source of error is reading and booking error and also weather conditions. Source of reading error is the sighting the staff over too long a distance, when it becomes impossible to take accurate readings. It is , therefore, recommended that sighting distances should be limited to 50m but, where absolutely unavoidable, this may be increased to maximum of 100m. For weather conditions, when it windy will cause the level to vibrate and give rise to difficulties in holding the staff steady. In hot weather, the effect of refraction are serious and produce a shimmering effect near ground level. The reading cannot be read accurately.


QUESTION : EXPLAIN BASIC RULES IN PRACTICE WHEN CONDUCTING A LEVELING


1.     EXPLAIN BASIC RULES IN PRACTICE WHEN CONDUCTING A LEVELING
    Image result for reciprocal levelling
                Levelling is the process of measuring the difference in elevation between two or more points. In engineering surveying, levelling has many application and is used at all stages in construction projects from the initial site survey through the final setting out. In practice, it is possible to measure heights to better than a few millimeters when levelling  this precision  is more than adequate for height measurement on the majority of civil engineering project.
Image result for reciprocal levelling
The basic rules in practice when conducting a levelling fieldwork should be adhered to if many of the sources of error are to be avoided. Levelling should always start and finish  at points of known reduced level so that misclosures can be detected. When only one bench mark is available, levelling lines must be run in loops starting and finishing at the bench mark. 
Image result for reciprocal levelling
Where possible, all sights length should below 50m. The staff must be held vertically by suitable use of a circular bubble or by rocking the staff and notong the minimum reading. Backsight and fortsight length should be equal for each instrument position. For engineering application, many intermediate sight readings may be taken from each set- up. Under this circumstances it is important that the level has no more than a small collimation error. 
Image result for reciprocal levelling
Reading should book immediately after they are observed and important readings, particularly at change points, should be checked. The rise and fall method of reduction should used when heighting reference or change  points and the HPR method ( height of collimation) should be used for contouring , sectioning and setting out applications.
Image result for reciprocal levelling