GIS Help Clinic

Rowley Birkin QC wrote: »

Excellent post YAT. Straightens out a lot for me!

Just on the last sentence, is it okay to work on a project where a number of shapefiles have different projections but all sit on top of each other due to the "on the fly" projection carried out by ArcMap? In terms of getting correct measurements, showing accurate scales etc. or will I run into big issues down the line? Sorry again if this is a silly question.

I have a considerable amount of work done for the project I'm working on but thinking of starting again as I've been tinkering around and managed to get all of my layers to sit on top of each other for the first time!

It is entirely possible that the different techniques may use slightly different parameters for the transformation of data from one coordinate system to another. As a result, the different methods may throw up slightly different results. If you try it out you may see a slight discrepancy between a file permanently projected using ArcToolbox and a file that is projected on-the-fly by ArcMap.

The geoprocessing tools provided by ArcToolbox are entirely separate from the ArcMap tools that do the same job, ie their programming is different. When you do a buffer operation using Analysis Tools-Proximity-Buffer the code that is run to carry out the operation is different to that when you use Editor-Buffer in ArcMap.

The same is true when carrying out projections. However, these projections require transformation parameters. The different techniques may use parameters of varying precision, providing slightly different results.

Personally, when doing GIS analysis work, I always preferred to project all data to a standard project coordinate system, say Irish National Grid. To put it another way I never used on-the-fly projection and never mixed coordinate systems in ArcMap. I would also always be consistent in the technique used, ie I always used ArcToolbox for projecting.

However, if you're just making a map for display purposes I wouldn't be too worried.

Rowley Birkin QC wrote: »

Cheers again for the answer YAT.

The project I'm working on requires me to come up with some calculations such as energy used per km2 or similar. Data gathering aside I've been thinking of ways to display this in the GIS. So far I've downloaded an extension for ArcMap (ET GeoWizards) and used that to draw a grid on top of my map made up of polygons measuring 2.5km2. After that I'm a bit stumped.

Ideally I'd like to be able to take my data, click on a grid polygon with the info cursor and be able to call up the figures for that given area. I know it's possible but just wondering how I would go about it.

You say that you are interested in displaying "energy used per sq km". Before you go any further you would need to have a sourced this "energy used" data. Have you got that far yet? If yes then is it in the form of discrete features, ie points, or is it a continuous surface where each cell has an "energy used" value?

Rowley Birkin QC wrote: »

I have sourced the data by way of a survey of 50 businesses within the study area. However confidentiality was a condition of the businesses volunteering the information so using points is probably out as the businesses will be identifiable.

In that case a continuous surface where each sq km has a value seems to be the way to go.

First of all I don't believe a continuous surface would be suitable. The measured value here, "Energy Used", is not a function of its XY location. Unlike height, you can't interpolate the "Energy Used" value at a specific location based on known values at other adjacent locations. It would be like interpolating the population of Athlone based on the populations of Galway and Dublin.

A better approach maybe to have your study area divided into separate geographic entities (you mentioned using a grid although I'm not sure how that would work). Say the study area is Limerick City, suitable subdivisions of the city may be the city's Electoral Divisions.

1. Download the EDs from the CSO website and then unzip.

2. Navigate to the shapefile location in ArcCatalog. Right click - Properties and set the XY Coordinate System to Irish National Grid.

3. Add the shapefile to ArcMap. Next you want to select out just those polygons located in Limerick city. Selection - Select By Attributes.
The SQL code to select all EDs in the city of Limerick is "County" = 'Limerick City'

4. When selected, right click on the layer and choose Data - Export Data to save the selected features as a new shapefile. Call it Limerick_EDs.shp for instance.

5. Now you have polygons that cover the project area. As you will be normalizing data by area later it's best to add a suitable field now.

Open the table and choose Options - Add Field.
For Name input AreaSqKm
For Type input Double

6. Right click on this new field, select Calculate Geometry and ignore the warning message.
For Property choose Area
For Units choose Square Kilometers [sq km]

The polygon layer is now ready. I'm fully aware that the Electoral Divisions I'm using here will probably be unsuitable for you project but it gives you an idea.


To replicate your survey data I created 100 randomly located points within the study area. Each point has a random integer value between 0 and 100 representing ""Energy Used".



The next stage will help maintain confidentiality by taking the "Energy Used" attribute of each business and applying it to the polygon within which it is located. This way we can get total, average, maximum, minimum values for each polygon.

1. Right click on Limerick_EDs in the Table of Contents. Select Joins and Relates - Join

For What do you want to join to this layer? select Join data from another layer based on spatial location. (This is called a spatial join)

2. For 1 select Energy_Used (ie the points layer)
For 2 ensure that Average, Sum, Minimum and Maximum are checked
For 3 choose a suitable location and call the output Limerick_EnergyUsed

When completed open the new layers table. You will see that each polygon within the project area has
1) A name, Saps_Label
2) An area value in square kilometers, AreaSqKm
3) A count of the number of points that lay within that individual polygon, Count.
4) The sum of the Energy value of all points that lay within that polygon, Sum_Energy.
5) The average of those Energy values, Avg_Energy.
6) The smallest of those values, Min_Energy.
7) The largest of those, Max Energy.


What I want is to map the Average Energy Consumption of each Electoral Division per Square Kilometer.

1. Right click on the layer Limerick_EnergyUsed, select Properties and choose the Symbology tab.

2. Under Show select Quantities-Graduated Colors.

3. For Fields - Value choose Avg_Energy

4. For Fields - Normalization choose AreaSqKm

5. A total of five classes are automatically created. However, the data is skewed by 0 values, ie those polygons inside of which no survey data was present.

6. Select Classify, select Exclusion, input "Avg_Energy" = 0 and click OK.

7. You can now modify the number of classes required and choose a suitable Color Ramp. The result is the Electoral Divisions classified by Average Energy Consumption per square kilometer.




Alternatively you could create a permanent record of this data.

1. Add a new field called AvgEngKM of type double.
2. In Field Calculator input [Avg_Energy] / [AreaSqKm]

Now when you use the identify tool on an individual Electoral District you can see all the associated data including the Average Energy Consumption per Square Kilometer.


It's difficult to get a handle on what exactly it is you're trying to do but I hope this helps.

Cork_girl wrote:

I'm trying to group Corine files into groups such as general Forestry, water bodies, etc. but when I go to data, export I can't export as a .dbf - the only option that will work is as a .shp?
The file I'm working with is one I have exported before - as I have edited it down to just certain areas etc.
Am I missing something?!

What is the source filetype you are trying to export?

If you are trying to do Step2 of the quoted text and export a csv to dbf then .shp will never be an available option. How could it?

If instead you are trying to export only the attribute data of a shapefile layer to a dbf then do the following

1. Right click on the layer and select Open Attribute Table
2. Click Options - Export. The Export Data dialog will open.
3. Click the Browse button, ie the icon that looks like an open folder.
4. For Save as type: ensure dBASE Table is selected.

Hi guys, I'm trying to find a way to get heights in corporated into my map also, basically I just need to find the tops of the mountains on my map. Basically what I want to be able to do is have a height parameter included in my attribute table and a way to tell the highest point - mainly just the last one but the rest would be great too.

I've had a look online and it says that there should be something in my properties tab that says Base Heights, but as you can see from the attachment I don't have that. Any ideas what I can do? I have contours already and I've got slope, but I can figure out how to use them to tell the height of an area when I use the information pointer.

Thank you in anticipation.

Rowley Birkin QC wrote: »

Just to go back to this YAT, if the study area comprised just one Electoral Divison is there a way to subdivide that further without actually physically drawing the polygons and following the same procedure as you outlined above?

You should repeat the process if you are introducing smaller polygons than used initially.

First of all the data for this single Electoral Division is a summarised version of the original survey data. If you wanted to split this polygon in two and redefine the numerical attributes of the new polygons to be proportional to the area of the original, then you are introducing unnecessary error.

For example, if the original polygon is split 75/25 and the attributes are redefined accordingly then the AverageValue, TotalValue,
MinValue, MaxValue of the larger polygon will be simply give a value of 75% of the original polygons values. Obviously in this case that makes no sense.

You can easily split a polygon using the Editor menu, however the new polygons will inherit the attributes of the parent polygon.

You could also intersect your polygon with other features but there would also be issues with this method.

So you're better off repeating the process if you need to subdivide the study area.

WesternZulu wrote: »

Any help would be appreciated.

I am using the electoral boundary cso data http://beyond2020.cso.ie/censusasp/saps/boundaries/ED%20Disclaimer1.htm

I have downloaded arcgis explorer but cannot open any of the shapefiles, it either comes up as "unknown coordinates" or "invalid file" when i try and add them from the add content option.

I have never used any GIS programme before and this is wrecking my head that i cannot even view the files never mind get anything else done with them.

Is this a common problem or am i not doing something obvious

You need all three components of this shapefile to be present in a single folder for it to be valid and readable. The shp component contains the geometry of the file, ie the lines, points or whatever. The dbf stores the attribute information of features in the shp. The shx is an index, which aids in the speed with which the file can be used. Make sure all three components are together before you try to use it.

If that is not the problem then it may be because the file doesn't have a prj component. This tells the software what map projection the data is in.

I have created a prj file for this shapefile, you can download it here. Place it in the same folder as the other three components and try to add the data again.

WesternZulu wrote: »

Thanks for that. Appreciate it!

I think that my netbook is unable to run arcgis explorer properly so that could be a reason too.

Have installed it and reinstalled it but the same problem of the not being ble to see any map in the screen happens.

All i can see it whatever window is opened behind arcgis explorer where the map should appear.
I'm getting a headache trying to sort this out :eek:
Not the easiest programme in the world to work

What OS are you using? To run properly ArcGIS Explorer needs Service Pack 2 for Windows XP or Service Pack 1 or 2 for Vista.

lil*lady wrote: »

Bingo, got it, thank you, feel quite silly now.

Another silly question would be, what would I put in though to get the heights? Sorry this must be like pulling teeth, but the slope file is a "floating point raster" and it won't allow me to get data from that. Thanks again.

The slope file doesn't contain the necessary height information in any case. Remember that the values associated with each pixel will be slope, not height. Use the surface from which the slope file was derived, this contains the necessary height information.

lil*lady wrote: »

Thanks, the only thing is that when I'm trying to create a new feature it says that it exceeds the 2gb limit and won't let me get any further, is there any way around this?

You have two options.

The first involves reducing the raster layer resolution. Right click on the elevation surface layer name in the TOC. Select Data-Export Data, the Export Raster Data dialog will open. For Cell Size (cx, cy) input a suitably larger value.

For example, if the original values were 10, 10, this means that each pixel covers 10 square meters on the ground. As you increase the Cell Size note that the Uncompressed Size property underneath will decrease. This is the size of the output file. You can continue to incrementally increase the Cell Size until an acceptable file size is reached. Set the Name, Location and Format options and then Save.

Alternatively, if you want to maintain the original resolution as the grid interval in your output features you could split the original raster into smaller components. First zoom over the top left quadrant of the raster. Right click on the elevation surface layer name in the TOC. Select Data-Export Data, the Export Raster Data dialog will open. Under Extent select Data Frame (Current). Set the Name, Location and Format options and then Save. Repeat for the remaining quadrants. Using this method you will end up with four rasters but each will be 1/4 the file size of the original.

In both cases note the Format option. If you are working with GRID files make sure you set this option to GRID.

YoureATowel firstly thank you for all your help so far, I wouldn't be as far along as I am if it wasn't for your help. And I'm hoping you'll help with this one other thing. It's still the elevation problem.

The original file that I've gotten my slope etc from is an image file. I figured out that the pixel value is the elevation, by clicking the information button randomly on the image actually. I can't get it to open in any way that'd be like an attribute table for me to just click and join it. I've tried to change the image to a raster to get it to the raster - > features part that you suggested before but it won't work. I've also tried to classify it, but that won't work. And I looked at getting the height data from the contours but it stops at 460m and I know for a fact that the highest peak is 466m, but I can't get Arc to tell me that won't tell that.

Is there anyway that I could get a table of the pixel values (there's no attribute table, classifying unique values in the symbology doesn't seem to work) and convert them to elevation?

Rowley Birkin QC wrote: »

Any ideas how to do that with an MrSid image Youreatowel?

I tried the procedure you outlined above, it worked but the output file is blank despite the fact it is 500MB + in size. The original MrSID image it was cropped from is over 4GB.

If you did it as outlined then you should not have run into any problems. When you selected Data Frame (Current) were you sure that the ArcMap data frame did indeed contain imagery?

What output format did you use? If it was .img then any space in the data frame not filled by your original image will be given all 0 RGB values, ie black. If no imagery exists in the data frame then you will just get a blank .img file.

lil*lady wrote: »

Hi YourATowel, I'm fairly sure it's the elevation, the data corresponds to previously known data and the contours. I've put a link to my original data, it's a lidar image of what I've been working off and what everything's based off, if you have time if you could have a look I'd really appreciate it, if you don't that's perfectly fine. I still can't convert it to point features, I'm still having the same problems and I can't get curvature to work either, I can get a curvature output but then there are classification problems, it freezes if I try and do it through symbology or reclassify

Thanks again for all your help.

http://rapidshare.com/files/417383889/Clare_Island.zip

OK, the problem is that the raster dataset is of a 32 bit floating point pixel type. Such raster types don't support raster attribute tables in ArcGIS, as a result many processing tasks are not possible. The solution is to then convert it to a usable format, ie a raster with a single integer band.

1. Open ArcCatalog and navigate to the folder containing the raster in question.
2. Right click on it, select Export - Raster To Different Format. The Copy Raster dialog will open.
3. Give the output a distinct name.
4. Most importantly, under Pixel Type (optional), select either 16_Bit_Signed or 16_Bit_Unsigned. Either option is suitable in this instance.
5. Click OK.

The output cell values will be rounded to the nearest whole number, 99.44 becomes 99 while 99.55 becomes 100.

At this point it would be wise to reduce the raster layer resolution before continuing to create the regular grid of points. Using this reduced resolution raster for your other tasks will not only help with processing time but will also ensure that the application doesn't freeze.

lil*lady wrote: »

Thanks YoureATowel, got it working now. Really happy, thank you.

I've just one final question. I've made a curvature raster as well, the only thing I can't figure out is how do I classify it so that I know what is convex and concave etc? I just need to know up to what value is convex, what's flat, what's concave.

The range in values is from -855.644 to 1546.25 for the curvature profile and -2830 to 1640 for the curvature.

Thanks once again.

For the curvature, positive values indicate that the surface is convex, negative values indicate that that cell is concave and 0 means that it is flat.

For the profile curvature the opposite is true. Positive = concave, negative = convex.

However, the range of values you provided indicate that there are anomalous values at the extremities of the range.

Thanks YoureATowel, you've been an immense help. I'll play around with it a bit and hopefully it'll work out.

I have one final final question if you can answer YoureATowel or anyone else I'd really appreciate it, in the home stretch now.

I'm submitting my map shortly, however I tested out the map on a different computer today and I realised that all the symbology isn't set up with it, which is fairly important.

As I've created my own symbology and certain polygons have very specific colours, I was wondering how do I create a style file or something establishes what the symbology is when it is opened on another computer?

Edit: I think I have it, created a new folder in the style manager section and just copied everything to there, won't be at another computer until Thursday so I won't be able to tell if it's working til then. Last thing is, in the style manager, can I assign a particular field in the attribute table to the symbol, currently it's coming up as default..or is it just a case that when you go into arcgis with the map my prof is going to have to click-symbology-and then choose the symbology...? Really hope that's not the case...would love if there was someway I could just program it now

lil*lady wrote: »

I have one final final question if you can answer YoureATowel or anyone else I'd really appreciate it, in the home stretch now.

I'm submitting my map shortly, however I tested out the map on a different computer today and I realised that all the symbology isn't set up with it, which is fairly important.

As I've created my own symbology and certain polygons have very specific colours, I was wondering how do I create a style file or something establishes what the symbology is when it is opened on another computer?

Edit: I think I have it, created a new folder in the style manager section and just copied everything to there, won't be at another computer until Thursday so I won't be able to tell if it's working til then. Last thing is, in the style manager, can I assign a particular field in the attribute table to the symbol, currently it's coming up as default..or is it just a case that when you go into arcgis with the map my prof is going to have to click-symbology-and then choose the symbology...? Really hope that's not the case...would love if there was someway I could just program it now

Not entirely sure what you're saying here. When you save a mxd file all the styles are saved within it. If you then open that mxd file on another computer ArcMap will add the layers and restyle them as specified by you at the time the project was saved. Only in certain situations should there be problems, like if you used a bmp for a point style, the path of that bmp must be available to the computer opening the mxd.

Would it not be best to just output a pdf of your finished map?

lil*lady wrote: »

Also, I've an issue with a symbol that keeps resizing itself and I don't want it to, and ideas on how to keep it a certain size? (see PDF)
Thank you!

In this situation you have to set a Reference Scale for your data frame.

Go to View-Data Frame Properties and select a suitable reference scale for your map.

Next, right click on the layer in question in the TOC, select Properties and then the Display tab. For your purposes, ensure that the Scale symbols when a reference scale is set is unselected.

Now, if you zoom in and out, the symbols will remain the same size as they are at the selected Reference Scale.